diff --git a/composer.json b/composer.json
index 553590e..770c56c 100644
--- a/composer.json
+++ b/composer.json
@@ -16,6 +16,7 @@
   "require-dev": {
     "lunr/halo": "dev-master",
     "phpunit/phpunit": ">=9.0 <9.6",
+    "mockery/mockery": "~1.5",
     "ext-xdebug": ">=3.1"
   },
   "config": {
diff --git a/decomposer.json b/decomposer.json
index 8a767af..1145b58 100644
--- a/decomposer.json
+++ b/decomposer.json
@@ -55,5 +55,13 @@
             "prefix": "WpOrg\\Requests",
             "search-path": "/src/"
         }
+    },
+    "Mockery": {
+        "url": "https://github.com/mockery/mockery.git",
+        "version": "1.6.7",
+        "psr0": {
+            "path": "/library/"
+        },
+        "development-only": true
     }
 }
diff --git a/src/Lunr/Vortex/FCM/FCMDispatcher.php b/src/Lunr/Vortex/FCM/FCMDispatcher.php
index adc61a1..0914819 100644
--- a/src/Lunr/Vortex/FCM/FCMDispatcher.php
+++ b/src/Lunr/Vortex/FCM/FCMDispatcher.php
@@ -10,9 +10,18 @@
 
 namespace Lunr\Vortex\FCM;
 
+use BadMethodCallException;
+use DateTimeImmutable;
 use InvalidArgumentException;
+use Lcobucci\JWT\Encoding\ChainedFormatter;
+use Lcobucci\JWT\Encoding\JoseEncoder;
+use Lcobucci\JWT\Signer\Key\InMemory;
+use Lcobucci\JWT\Signer\Rsa\Sha256;
+use Lcobucci\JWT\Token\Builder;
 use Lunr\Vortex\PushNotificationDispatcherInterface;
 use Psr\Log\LoggerInterface;
+use RuntimeException;
+use UnexpectedValueException;
 use WpOrg\Requests\Exception as RequestsException;
 use WpOrg\Requests\Response;
 use WpOrg\Requests\Session;
@@ -23,10 +32,28 @@
 class FCMDispatcher implements PushNotificationDispatcherInterface
 {
     /**
-     * Push Notification authentication token.
+     * Push Notification Oauth token.
      * @var string
      */
-    protected string $auth_token;
+    protected ?string $oauth_token;
+
+    /**
+     * FCM id of the project.
+     * @var ?string
+     */
+    protected ?string $project_id;
+
+    /**
+     * FCM client email of the project.
+     * @var ?string
+     */
+    protected ?string $client_email;
+
+    /**
+     * FCM id of the project.
+     * @var ?string
+     */
+    protected ?string $private_key;
 
     /**
      * Shared instance of the Requests\Session class.
@@ -45,7 +72,19 @@ class FCMDispatcher implements PushNotificationDispatcherInterface
      * Url to send the FCM push notification to.
      * @var string
      */
-    private const GOOGLE_SEND_URL = 'https://fcm.googleapis.com/fcm/send';
+    private const GOOGLE_SEND_URL = 'https://fcm.googleapis.com/v1/projects/';
+
+    /**
+     * Url to fetch the OAuth2 token.
+     * @var string
+     */
+    private const GOOGLE_OAUTH_URL = 'https://oauth2.googleapis.com/token';
+
+    /**
+     * Default lifetime for the OAuth token.
+     * @var string
+     */
+    private const DEFAULT_OAUTH_LIFETIME = '+10 minutes';
 
     /**
      * Constructor.
@@ -55,9 +94,12 @@ class FCMDispatcher implements PushNotificationDispatcherInterface
      */
     public function __construct(Session $http, LoggerInterface $logger)
     {
-        $this->http       = $http;
-        $this->logger     = $logger;
-        $this->auth_token = '';
+        $this->http         = $http;
+        $this->logger       = $logger;
+        $this->oauth_token  = NULL;
+        $this->project_id   = NULL;
+        $this->client_email = NULL;
+        $this->private_key  = NULL;
     }
 
     /**
@@ -65,11 +107,86 @@ public function __construct(Session $http, LoggerInterface $logger)
      */
     public function __destruct()
     {
-        unset($this->auth_token);
+        unset($this->oauth_token);
+        unset($this->project_id);
+        unset($this->client_email);
+        unset($this->private_key);
         unset($this->http);
         unset($this->logger);
     }
 
+    /**
+     * Set the FCM project id for sending notifications.
+     *
+     * @param string $project_id The id of the FCM project
+     *
+     * @return $this
+     */
+    public function set_project_id(string $project_id): static
+    {
+        $this->project_id = $project_id;
+
+        return $this;
+    }
+
+    /**
+     * Set the FCM client email for sending notifications.
+     *
+     * @param string $client_email The client email of the FCM project
+     *
+     * @return $this
+     */
+    public function set_client_email(string $client_email): static
+    {
+        $this->client_email = $client_email;
+
+        return $this;
+    }
+
+    /**
+     * Set the FCM private key for sending notifications.
+     *
+     * @param string $private_key The private key of the FCM project
+     *
+     * @return $this
+     */
+    public function set_private_key(string $private_key): static
+    {
+        $this->private_key = $private_key;
+
+        return $this;
+    }
+
+    /**
+     * Set a token to authenticate with.
+     *
+     * @param string $token The OAuth token to use
+     *
+     * @return $this
+     */
+    public function set_oauth_token(string $token): static
+    {
+        $this->oauth_token = $token;
+
+        return $this;
+    }
+
+    /**
+     * Request and set an oauth token from FCM.
+     *
+     * @param string $oauth_lifetime Relative time as a string for strtotime() to parse into an expiry timestamp.
+     *
+     * @see https://www.php.net/manual/en/datetime.formats.php#datetime.formats.relative
+     *
+     * @return $this
+     */
+    public function configure_oauth_token(string $oauth_lifetime = self::DEFAULT_OAUTH_LIFETIME): static
+    {
+        $this->set_oauth_token($this->get_oauth_token($oauth_lifetime));
+
+        return $this;
+    }
+
     /**
      * Getter for FCMResponse.
      *
@@ -105,9 +222,25 @@ public function push(object $payload, array &$endpoints): FCMResponse
             throw new InvalidArgumentException('No endpoints provided!');
         }
 
+        if ($this->oauth_token === NULL)
+        {
+            $context = [ 'endpoint' => $endpoints[0] ];
+            $this->logger->warning('Tried to push FCM notification to {endpoint} but wasn\'t authenticated.', $context);
+
+            return $this->get_response($this->get_new_response_object_for_failed_request(401), $this->logger, $endpoints[0], $payload->get_payload());
+        }
+
+        if ($this->project_id === NULL)
+        {
+            $context = [ 'endpoint' => $endpoints[0] ];
+            $this->logger->warning('Tried to push FCM notification to {endpoint} but project id is not provided.', $context);
+
+            return $this->get_response($this->get_new_response_object_for_failed_request(400), $this->logger, $endpoints[0], $payload->get_payload());
+        }
+
         $headers = [
             'Content-Type'  => 'application/json',
-            'Authorization' => 'key=' . $this->auth_token,
+            'Authorization' => 'Bearer ' . $this->oauth_token,
         ];
 
         $tmp_payload = json_decode($payload->get_payload(), TRUE);
@@ -123,7 +256,7 @@ public function push(object $payload, array &$endpoints): FCMResponse
                 'connect_timeout' => 15 // timeout in seconds
             ];
 
-            $http_response = $this->http->post(self::GOOGLE_SEND_URL, $headers, $json_payload, $options);
+            $http_response = $this->http->post(self::GOOGLE_SEND_URL . $this->project_id . '/messages:send', $headers, $json_payload, $options);
         }
         catch (RequestsException $e)
         {
@@ -144,29 +277,103 @@ public function push(object $payload, array &$endpoints): FCMResponse
     }
 
     /**
-     * Set the the auth token for the http headers.
+     * Get the oauth token for the http headers.
      *
-     * @param string $auth_token The auth token for the fcm push notifications
+     * @param string $oauth_lifetime Relative time as a string for strtotime() to parse into an expiry timestamp
      *
-     * @return FCMDispatcher Self reference
+     * @see https://www.php.net/manual/en/datetime.formats.php#datetime.formats.relative
+     *
+     * @return string The OAuth_token
      */
-    public function set_auth_token(string $auth_token): self
+    public function get_oauth_token(string $oauth_lifetime = self::DEFAULT_OAUTH_LIFETIME): string
     {
-        $this->auth_token = $auth_token;
+        if (strtotime($oauth_lifetime) === FALSE)
+        {
+            throw new InvalidArgumentException('Invalid oauth lifetime!');
+        }
 
-        return $this;
+        if ($this->client_email === NULL)
+        {
+            throw new BadMethodCallException('Requesting token failed: No client email provided');
+        }
+
+        if ($this->private_key === NULL)
+        {
+            throw new BadMethodCallException('Requesting token failed: No private key provided');
+        }
+
+        $issued_at = new DateTimeImmutable();
+
+        $token_builder = new Builder(new JoseEncoder(), ChainedFormatter::default());
+
+        $token = $token_builder->issuedBy($this->client_email)
+                               ->permittedFor('https://oauth2.googleapis.com/token')
+                               ->issuedAt($issued_at)
+                               ->expiresAt($issued_at->modify($oauth_lifetime))
+                               ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                               ->withHeader('alg', 'RS2256')
+                               ->withHeader('typ', 'JWT')
+                               ->getToken(new Sha256(), InMemory::plainText($this->private_key));
+
+        $headers = [
+            'Content-Type' => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => $token->toString(),
+        ];
+
+        try
+        {
+            $http_response = $this->http->post(self::GOOGLE_OAUTH_URL, $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), []);
+        }
+        catch (RequestsException $e)
+        {
+            $context = [ 'message' => $e->getMessage() ];
+            $this->logger->warning('Fetching OAuth token for FCM notification(s) failed: {message}', $context);
+
+            throw new RuntimeException('Fetching OAuth token for FCM notification(s) failed', 0, $e);
+        }
+
+        $response_body = json_decode($http_response->body, TRUE);
+
+        if (json_last_error() !== JSON_ERROR_NONE)
+        {
+            $context = [ 'message' => json_last_error_msg() ];
+            $this->logger->warning('Processing json response for fetching OAuth token for FCM notification(s) failed: {message}', $context);
+
+            $message = 'Processing json response for fetching OAuth token for FCM notification(s) failed: ' . $context['message'];
+            throw new UnexpectedValueException($message);
+        }
+
+        if (!array_key_exists('access_token', $response_body))
+        {
+            $error_msg = $response_body['error_description'] ?? 'No access token in the response body';
+
+            $context = [ 'error' => $error_msg ];
+            $this->logger->warning('Fetching OAuth token for FCM notification(s) failed: {error}', $context);
+
+            throw new UnexpectedValueException('Fetching OAuth token for FCM notification(s) failed: ' . $error_msg);
+        }
+
+        return $response_body['access_token'];
     }
 
     /**
      * Get a Requests\Response object for a failed request.
      *
+     * @param int $http_code Set http code for the request.
+     *
      * @return Response New instance of a Requests\Response object.
      */
-    protected function get_new_response_object_for_failed_request(): Response
+    protected function get_new_response_object_for_failed_request(?int $http_code = NULL): Response
     {
         $http_response = new Response();
 
-        $http_response->url = self::GOOGLE_SEND_URL;
+        $http_response->url = self::GOOGLE_SEND_URL . $this->project_id . '/messages:send';
+
+        $http_response->status_code = $http_code ?? FALSE;
 
         return $http_response;
     }
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherBaseTest.php b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherBaseTest.php
index 4c2e9f9..47d75d8 100644
--- a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherBaseTest.php
+++ b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherBaseTest.php
@@ -31,11 +31,35 @@ public function testRequestsSessionIsSetCorrectly(): void
     }
 
     /**
-     * Test that the auth token is set to an empty string by default.
+     * Test that the OAuth token is set to null by default.
      */
-    public function testAuthTokenIsEmptyString(): void
+    public function testOAuthTokenIsSetToNull(): void
     {
-        $this->assertPropertyEquals('auth_token', '');
+        $this->assertPropertyEquals('oauth_token', NULL);
+    }
+
+    /**
+     * Test that the project_id is set to null by default.
+     */
+    public function testProjectIdIsSetToNull(): void
+    {
+        $this->assertPropertyEquals('project_id', NULL);
+    }
+
+    /**
+     * Test that the client_email is set to null by default.
+     */
+    public function testClientEmailIsSetToNull(): void
+    {
+        $this->assertPropertyEquals('client_email', NULL);
+    }
+
+    /**
+     * Test that the private_key is set to null by default.
+     */
+    public function testPrivateKeyIsSetToNull(): void
+    {
+        $this->assertPropertyEquals('private_key', NULL);
     }
 
     /**
@@ -45,12 +69,14 @@ public function testAuthTokenIsEmptyString(): void
      */
     public function testGetNewResponseObjectForFailedRequest(): void
     {
+        $this->set_reflection_property_value('project_id', 'fcm-project');
+
         $method = $this->get_accessible_reflection_method('get_new_response_object_for_failed_request');
 
         $result = $method->invoke($this->class);
 
         $this->assertInstanceOf('WpOrg\Requests\Response', $result);
-        $this->assertEquals('https://fcm.googleapis.com/fcm/send', $result->url);
+        $this->assertEquals('https://fcm.googleapis.com/v1/projects/fcm-project/messages:send', $result->url);
     }
 
     /**
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherConfigureOAuthTokenTest.php b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherConfigureOAuthTokenTest.php
new file mode 100644
index 0000000..ce2713a
--- /dev/null
+++ b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherConfigureOAuthTokenTest.php
@@ -0,0 +1,570 @@
+<?php
+
+/**
+ * This file contains the FCMDispatcherConfigureOAuthTokenTest class.
+ *
+ * SPDX-FileCopyrightText: Copyright 2024 Move Agency Group B.V., Zwolle, The Netherlands
+ * SPDX-License-Identifier: MIT
+ */
+
+namespace Lunr\Vortex\FCM\Tests;
+
+use BadMethodCallException;
+use DateTimeImmutable;
+use InvalidArgumentException;
+use Lcobucci\JWT\Token\Builder;
+use Mockery;
+use RuntimeException;
+use UnexpectedValueException;
+use WpOrg\Requests\Exception as RequestsException;
+use WpOrg\Requests\Response;
+
+/**
+ * This class contains tests for the setters of the FCMDispatcher class.
+ *
+ * @covers Lunr\Vortex\FCM\FCMDispatcher
+ */
+class FCMDispatcherConfigureOAuthTokenTest extends FCMDispatcherTest
+{
+
+    /**
+     * Test that configure_oauth_token() fails
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenThrowsInvalidArgumentException(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Invalid oauth lifetime!');
+
+        $this->token_builder->shouldReceive('issuedBy')
+                            ->never();
+
+        $this->http->expects($this->never())
+                   ->method('post');
+
+        $this->logger->expects($this->never())
+                     ->method('warning');
+
+        $this->class->configure_oauth_token('invalid relative time');
+    }
+
+    /**
+     * Test configure_oauth_token fails when client_email is NULL.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenFailsWhenClientEmailIsNull(): void
+    {
+        $this->expectException(BadMethodCallException::class);
+        $this->expectExceptionMessage('Requesting token failed: No client email provided');
+
+        $this->token_builder->shouldReceive('issuedBy')
+                            ->never();
+
+        $this->http->expects($this->never())
+                   ->method('post');
+
+        $this->logger->expects($this->never())
+                     ->method('warning');
+
+        $this->class->configure_oauth_token();
+
+        $this->assertPropertyEquals('oauth_token', NULL);
+    }
+
+    /**
+     * Test configure_oauth_token fails when private_key is NULL.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenFailsWhenPrivateKeyIsNull(): void
+    {
+        $this->set_reflection_property_value('client_email', 'email_client');
+
+        $this->expectException(BadMethodCallException::class);
+        $this->expectExceptionMessage('Requesting token failed: No private key provided');
+
+        $this->token_builder->shouldReceive('issuedBy')
+                            ->never();
+
+        $this->http->expects($this->never())
+                   ->method('post');
+
+        $this->logger->expects($this->never())
+                     ->method('warning');
+
+        $this->class->configure_oauth_token();
+
+        $this->assertPropertyEquals('oauth_token', NULL);
+    }
+
+    /**
+     * Test configure_oauth_token when fetching token fails.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenWhenFetchingTokenFails(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Fetching OAuth token for FCM notification(s) failed');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willThrowException(new RequestsException('cURL error 10: Request error', 'curlerror', NULL));
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with('Fetching OAuth token for FCM notification(s) failed: {message}', [ 'message' => 'cURL error 10: Request error' ]);
+
+        $this->class->configure_oauth_token('+10 minutes');
+
+        $this->assertPropertyEquals('oauth_token', NULL);
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test configure_oauth_token when processing json response fails.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenWhenProcessingJsonResponseFails(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $this->expectException(UnexpectedValueException::class);
+        $this->expectExceptionMessage('Processing json response for fetching OAuth token for FCM notification(s) failed: Syntax error');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = '{';
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with(
+                         'Processing json response for fetching OAuth token for FCM notification(s) failed: {message}',
+                         [ 'message' => 'Syntax error' ]
+                     );
+
+        $this->class->configure_oauth_token();
+
+        $this->assertPropertyEquals('oauth_token', NULL);
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test configure_oauth_token when processing response fails with general error.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenFailsWithGeneralError(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $this->expectException(UnexpectedValueException::class);
+        $this->expectExceptionMessage('Fetching OAuth token for FCM notification(s) failed: No access token in the response body');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = '{"token":"oauth_token1"}';
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with(
+                         'Fetching OAuth token for FCM notification(s) failed: {error}',
+                         [ 'error' => 'No access token in the response body' ]
+                     );
+
+        $this->class->configure_oauth_token();
+
+        $this->assertPropertyEquals('oauth_token', NULL);
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test Get_oauth_token when processing response fails with upstream error.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenFailsWithUpstreamError(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $content   = file_get_contents(TEST_STATICS . '/Vortex/fcm/oauth_error.json');
+        $error_msg = json_decode($content, TRUE)['error_description'];
+
+        $this->expectException(UnexpectedValueException::class);
+        $this->expectExceptionMessage('Fetching OAuth token for FCM notification(s) failed: ' . $error_msg);
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = $content;
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with(
+                         'Fetching OAuth token for FCM notification(s) failed: {error}',
+                         [ 'error' => $error_msg ]
+                     );
+
+        $this->class->configure_oauth_token();
+
+        $this->assertPropertyEquals('oauth_token', NULL);
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test configure_oauth_token when fetching token succeeds.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::configure_oauth_token
+     */
+    public function testConfigureOAuthTokenWhenFetchingTokenSucceeds(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = '{"access_token":"oauth_token1"}';
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->class->configure_oauth_token();
+
+        $this->assertPropertyEquals('oauth_token', 'oauth_token1');
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+}
+
+?>
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherGetOAuthTokenTest.php b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherGetOAuthTokenTest.php
new file mode 100644
index 0000000..01cbfe9
--- /dev/null
+++ b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherGetOAuthTokenTest.php
@@ -0,0 +1,556 @@
+<?php
+
+/**
+ * This file contains the FCMDispatcherGetOAuthTokenTest class.
+ *
+ * SPDX-FileCopyrightText: Copyright 2024 Move Agency Group B.V., Zwolle, The Netherlands
+ * SPDX-License-Identifier: MIT
+ */
+
+namespace Lunr\Vortex\FCM\Tests;
+
+use BadMethodCallException;
+use DateTimeImmutable;
+use InvalidArgumentException;
+use Lcobucci\JWT\Token\Builder;
+use Mockery;
+use RuntimeException;
+use UnexpectedValueException;
+use WpOrg\Requests\Exception as RequestsException;
+use WpOrg\Requests\Response;
+
+/**
+ * This class contains tests for the setters of the FCMDispatcher class.
+ *
+ * @covers Lunr\Vortex\FCM\FCMDispatcher
+ */
+class FCMDispatcherGetOAuthTokenTest extends FCMDispatcherTest
+{
+
+    /**
+     * Test that get_oauth_token() fails
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenThrowsInvalidArgumentException(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Invalid oauth lifetime!');
+
+        $this->token_builder->shouldReceive('issuedBy')
+                            ->never();
+
+        $this->http->expects($this->never())
+                   ->method('post');
+
+        $this->logger->expects($this->never())
+                     ->method('warning');
+
+        $this->class->get_oauth_token('invalid relative time');
+    }
+
+    /**
+     * Test get_oauth_token fails when client_email is NULL.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenFailsWhenClientEmailIsNull(): void
+    {
+        $this->expectException(BadMethodCallException::class);
+        $this->expectExceptionMessage('Requesting token failed: No client email provided');
+
+        $this->token_builder->shouldReceive('issuedBy')
+                            ->never();
+
+        $this->http->expects($this->never())
+                   ->method('post');
+
+        $this->logger->expects($this->never())
+                     ->method('warning');
+
+        $this->class->get_oauth_token();
+    }
+
+    /**
+     * Test get_oauth_token fails when private_key is NULL.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenFailsWhenPrivateKeyIsNull(): void
+    {
+        $this->set_reflection_property_value('client_email', 'email_client');
+
+        $this->expectException(BadMethodCallException::class);
+        $this->expectExceptionMessage('Requesting token failed: No private key provided');
+
+        $this->token_builder->shouldReceive('issuedBy')
+                            ->never();
+
+        $this->http->expects($this->never())
+                   ->method('post');
+
+        $this->logger->expects($this->never())
+                     ->method('warning');
+
+        $this->class->get_oauth_token();
+    }
+
+    /**
+     * Test get_oauth_token when fetching token fails.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenWhenFetchingTokenFails(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Fetching OAuth token for FCM notification(s) failed');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willThrowException(new RequestsException('cURL error 10: Request error', 'curlerror', NULL));
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with('Fetching OAuth token for FCM notification(s) failed: {message}', [ 'message' => 'cURL error 10: Request error' ]);
+
+        $this->class->get_oauth_token();
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test get_oauth_token when processing json response fails.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenWhenProcessingJsonResponseFails(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $this->expectException(UnexpectedValueException::class);
+        $this->expectExceptionMessage('Processing json response for fetching OAuth token for FCM notification(s) failed: Syntax error');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = '{';
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with(
+                         'Processing json response for fetching OAuth token for FCM notification(s) failed: {message}',
+                         [ 'message' => 'Syntax error' ]
+                     );
+
+        $this->class->get_oauth_token();
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test get_oauth_token when processing response fails with general error.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenFailsWithGeneralError(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $this->expectException(UnexpectedValueException::class);
+        $this->expectExceptionMessage('Fetching OAuth token for FCM notification(s) failed: No access token in the response body');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = '{"token":"oauth_token1"}';
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with(
+                         'Fetching OAuth token for FCM notification(s) failed: {error}',
+                         [ 'error' => 'No access token in the response body' ]
+                     );
+
+        $this->class->get_oauth_token();
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test get_oauth_token when processing response fails with upstream error.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenFailsWithUpstreamError(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $content   = file_get_contents(TEST_STATICS . '/Vortex/fcm/oauth_error.json');
+        $error_msg = json_decode($content, TRUE)['error_description'];
+
+        $this->expectException(UnexpectedValueException::class);
+        $this->expectExceptionMessage('Fetching OAuth token for FCM notification(s) failed: ' . $error_msg);
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = $content;
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->logger->expects($this->once())
+                     ->method('warning')
+                     ->with(
+                         'Fetching OAuth token for FCM notification(s) failed: {error}',
+                         [ 'error' => $error_msg ]
+                     );
+
+        $this->class->get_oauth_token();
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+    /**
+     * Test get_oauth_token when fetching token succeeds.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::get_oauth_token
+     */
+    public function testGetOAuthTokenWhenFetchingTokenSucceeds(): void
+    {
+        if (!extension_loaded('uopz'))
+        {
+            $this->markTestSkipped('The uopz extension is not available.');
+        }
+
+        $this->set_reflection_property_value('client_email', 'email_client');
+        $this->set_reflection_property_value('private_key', 'secret_key');
+
+        $issued_at  = Mockery::mock(DateTimeImmutable::class);
+        $expires_at = Mockery::mock(DateTimeImmutable::class);
+
+        $issued_at->expects()
+                  ->modify('+10 minutes')
+                  ->andReturn($expires_at);
+
+        uopz_set_mock(DateTimeImmutable::class, $issued_at);
+        uopz_set_mock(Builder::class, $this->token_builder);
+
+        $this->token_builder->expects()
+                            ->issuedBy('email_client')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->permittedFor('https://oauth2.googleapis.com/token')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->issuedAt($issued_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->expiresAt($expires_at)
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withClaim('scope', 'https://www.googleapis.com/auth/firebase.messaging')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('alg', 'RS2256')
+                            ->andReturnSelf();
+
+        $this->token_builder->expects()
+                            ->withHeader('typ', 'JWT')
+                            ->andReturnSelf();
+
+        uopz_set_return($this->token_builder::class, 'getToken', $this->token_plain);
+
+        $this->token_plain->expects($this->once())
+                          ->method('toString')
+                          ->willReturn('jwt_token');
+
+        $headers = [
+            'Content-Type'  => 'application/json'
+        ];
+
+        $payload = [
+            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
+            'assertion'  => 'jwt_token',
+        ];
+
+        $response = new Response();
+
+        $response->body = '{"access_token":"oauth_token1"}';
+
+        $this->http->expects($this->once())
+                   ->method('post')
+                   ->with('https://oauth2.googleapis.com/token', $headers, json_encode($payload, JSON_UNESCAPED_UNICODE), [])
+                   ->willReturn($response);
+
+        $this->assertSame('oauth_token1', $this->class->get_oauth_token());
+
+        uopz_unset_return($this->token_builder::class, 'getToken');
+        uopz_unset_mock(DateTimeImmutable::class);
+        uopz_unset_mock(Builder::class);
+    }
+
+}
+
+?>
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherPushTest.php b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherPushTest.php
index 473a4b0..7d923ea 100644
--- a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherPushTest.php
+++ b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherPushTest.php
@@ -60,7 +60,7 @@ public function testPushingWithUnsupportedPayloadThrowsException($payload): void
     }
 
     /**
-     * Test that push() returns FCMResponseObject.
+     * Test that push() throws exception when no endpoints are provided.
      *
      * @covers Lunr\Vortex\FCM\FCMDispatcher::push
      */
@@ -74,6 +74,73 @@ public function testPushWithEmptyEndpointsThrowException(): void
         $this->class->push($this->payload, $endpoints);
     }
 
+    /**
+     * Test that push() returns response when oauth_token is null.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::push
+     */
+    public function testPushWhenOAuthTokenIsNull(): void
+    {
+        $endpoints = [ 'endpoint' ];
+
+        $this->payload->expects($this->once())
+                      ->method('get_payload')
+                      ->willReturn('{"collapse_key":"abcde-12345"}');
+
+        $this->set_reflection_property_value('oauth_token', NULL);
+
+        $this->logger->expects($this->exactly(2))
+                     ->method('warning')
+                     ->withConsecutive(
+                        [
+                            'Tried to push FCM notification to {endpoint} but wasn\'t authenticated.',
+                            [ 'endpoint' => 'endpoint' ]
+                        ],
+                        [
+                            'Dispatching FCM notification failed for endpoint {endpoint}: {error}',
+                            [ 'endpoint' => 'endpoint', 'error' => 'Error with authentication' ]
+                        ]
+                    );
+
+        $result = $this->class->push($this->payload, $endpoints);
+
+        $this->assertInstanceOf('Lunr\Vortex\FCM\FCMResponse', $result);
+    }
+
+    /**
+     * Test that push() returns response when project_id is null.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::push
+     */
+    public function testPushWhenProjectIdIsNull(): void
+    {
+        $endpoints = [ 'endpoint' ];
+
+        $this->payload->expects($this->once())
+                      ->method('get_payload')
+                      ->willReturn('{"collapse_key":"abcde-12345"}');
+
+        $this->set_reflection_property_value('oauth_token', 'test');
+        $this->set_reflection_property_value('project_id', NULL);
+
+        $this->logger->expects($this->exactly(2))
+                     ->method('warning')
+                     ->withConsecutive(
+                        [
+                            'Tried to push FCM notification to {endpoint} but project id is not provided.',
+                            [ 'endpoint' => 'endpoint' ]
+                        ],
+                        [
+                            'Dispatching FCM notification failed for endpoint {endpoint}: {error}',
+                            [ 'endpoint' => 'endpoint', 'error' => 'Invalid JSON ()' ]
+                        ]
+                    );
+
+        $result = $this->class->push($this->payload, $endpoints);
+
+        $this->assertInstanceOf('Lunr\Vortex\FCM\FCMResponse', $result);
+    }
+
     /**
      * Test that push() resets the properties after a push.
      *
@@ -87,7 +154,8 @@ public function testPushResetsProperties(): void
                       ->method('get_payload')
                       ->willReturn('{"collapse_key":"abcde-12345"}');
 
-        $this->set_reflection_property_value('auth_token', 'auth_token');
+        $this->set_reflection_property_value('oauth_token', 'oauth_token');
+        $this->set_reflection_property_value('project_id', 'fcm-project');
 
         $response = $this->getMockBuilder('WpOrg\Requests\Response')->getMock();
 
@@ -97,7 +165,8 @@ public function testPushResetsProperties(): void
 
         $this->class->push($this->payload, $endpoints);
 
-        $this->assertPropertyEquals('auth_token', 'auth_token');
+        $this->assertPropertyEquals('oauth_token', 'oauth_token');
+        $this->assertPropertyEquals('project_id', 'fcm-project');
     }
 
     /**
@@ -107,16 +176,19 @@ public function testPushResetsProperties(): void
      */
     public function testPushWithFailedRequest(): void
     {
+        $this->set_reflection_property_value('oauth_token', 'oauth_token');
+        $this->set_reflection_property_value('project_id', 'fcm-project');
+
         $this->mock_function('curl_errno', function () { return 10; });
 
         $endpoints = [ 'endpoint' ];
 
         $headers = [
             'Content-Type'  => 'application/json',
-            'Authorization' => 'key=',
+            'Authorization' => 'Bearer oauth_token',
         ];
 
-        $url  = 'https://fcm.googleapis.com/fcm/send';
+        $url  = 'https://fcm.googleapis.com/v1/projects/fcm-project/messages:send';
         $post = '{"to":"endpoint"}';
 
         $options = [
@@ -150,16 +222,19 @@ public function testPushWithFailedRequest(): void
      */
     public function testPushWithTimeoutRequest()
     {
+        $this->set_reflection_property_value('oauth_token', 'oauth_token');
+        $this->set_reflection_property_value('project_id', 'fcm-project');
+
         $this->mock_function('curl_errno', function () { return 28; });
 
         $endpoints = [ 'endpoint' ];
 
         $headers = [
             'Content-Type'  => 'application/json',
-            'Authorization' => 'key=',
+            'Authorization' => 'Bearer oauth_token',
         ];
 
-        $url  = 'https://fcm.googleapis.com/fcm/send';
+        $url  = 'https://fcm.googleapis.com/v1/projects/fcm-project/messages:send';
         $post = '{"to":"endpoint"}';
 
         $options = [
@@ -195,16 +270,19 @@ public function testPushWithTimeoutRequest()
      */
     public function testPushRequestWithDefaultValues(): void
     {
+        $this->set_reflection_property_value('oauth_token', 'oauth_token');
+        $this->set_reflection_property_value('project_id', 'fcm-project');
+
         $endpoints = [ 'endpoint' ];
 
         $response = $this->getMockBuilder('WpOrg\Requests\Response')->getMock();
 
         $headers = [
             'Content-Type'  => 'application/json',
-            'Authorization' => 'key=',
+            'Authorization' => 'Bearer oauth_token',
         ];
 
-        $url  = 'https://fcm.googleapis.com/fcm/send';
+        $url  = 'https://fcm.googleapis.com/v1/projects/fcm-project/messages:send';
         $post = '{"to":"endpoint"}';
 
         $options = [
@@ -227,22 +305,23 @@ public function testPushRequestWithDefaultValues(): void
      */
     public function testPushRequestWithSingleEndpoint(): void
     {
+        $this->set_reflection_property_value('oauth_token', 'oauth_token');
+        $this->set_reflection_property_value('project_id', 'fcm-project');
+
         $endpoints = [ 'endpoint' ];
 
         $this->payload->expects($this->once())
                       ->method('get_payload')
                       ->willReturn('{"collapse_key":"abcde-12345"}');
 
-        $this->set_reflection_property_value('auth_token', 'auth_token');
-
         $response = $this->getMockBuilder('WpOrg\Requests\Response')->getMock();
 
         $headers = [
             'Content-Type'  => 'application/json',
-            'Authorization' => 'key=auth_token',
+            'Authorization' => 'Bearer oauth_token',
         ];
 
-        $url  = 'https://fcm.googleapis.com/fcm/send';
+        $url  = 'https://fcm.googleapis.com/v1/projects/fcm-project/messages:send';
         $post = '{"collapse_key":"abcde-12345","to":"endpoint"}';
 
         $options = [
@@ -265,22 +344,23 @@ public function testPushRequestWithSingleEndpoint(): void
      */
     public function testPushRequestWithMultibyteCharacters(): void
     {
+        $this->set_reflection_property_value('oauth_token', 'oauth_token');
+        $this->set_reflection_property_value('project_id', 'fcm-project');
+
         $endpoints = [ 'endpoint' ];
 
         $this->payload->expects($this->once())
                       ->method('get_payload')
                       ->willReturn('{"collapse_key":"abcde-12345","data":{"message":"凄い"}}');
 
-        $this->set_reflection_property_value('auth_token', 'auth_token');
-
         $response = $this->getMockBuilder('WpOrg\Requests\Response')->getMock();
 
         $headers = [
             'Content-Type'  => 'application/json',
-            'Authorization' => 'key=auth_token',
+            'Authorization' => 'Bearer oauth_token',
         ];
 
-        $url  = 'https://fcm.googleapis.com/fcm/send';
+        $url  = 'https://fcm.googleapis.com/v1/projects/fcm-project/messages:send';
         $post = '{"collapse_key":"abcde-12345","data":{"message":"凄い"},"to":"endpoint"}';
 
         $options = [
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherSetTest.php b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherSetTest.php
index 57fcdfa..9cd7250 100644
--- a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherSetTest.php
+++ b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherSetTest.php
@@ -10,6 +10,8 @@
 
 namespace Lunr\Vortex\FCM\Tests;
 
+use InvalidArgumentException;
+
 /**
  * This class contains tests for the setters of the FCMDispatcher class.
  *
@@ -19,25 +21,81 @@ class FCMDispatcherSetTest extends FCMDispatcherTest
 {
 
     /**
-     * Test that set_auth_token() sets the endpoint.
+     * Test that set_project_id() sets the project_id.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_project_id
+     */
+    public function testSetProjectIDSetsProjectId(): void
+    {
+        $this->class->set_project_id('project_id');
+
+        $this->assertPropertyEquals('project_id', 'project_id');
+    }
+
+    /**
+     * Test the return of set_project_id().
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_project_id
+     */
+    public function testSetProjectIdReturnsSelfReference(): void
+    {
+        $this->assertEquals($this->class, $this->class->set_project_id('project_id'));
+    }
+
+    /**
+     * Test that set_client_email() sets the client_email.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_client_email
+     */
+    public function testSetClientEmailSetsClientEmail(): void
+    {
+        $this->class->set_client_email('email');
+
+        $this->assertPropertyEquals('client_email', 'email');
+    }
+
+    /**
+     * Test the return of set_client_email().
      *
-     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_auth_token
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_client_email
      */
-    public function testSetAuthTokenSetsPayload(): void
+    public function testSetClientEmailReturnsSelfReference(): void
     {
-        $this->class->set_auth_token('auth_token');
+        $this->assertEquals($this->class, $this->class->set_client_email('email'));
+    }
 
-        $this->assertPropertyEquals('auth_token', 'auth_token');
+    /**
+     * Test that set_private_key() sets the private_key.
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_private_key
+     */
+    public function testSetPrivateKeySetsPrivateKey(): void
+    {
+        $this->class->set_private_key('key');
+
+        $this->assertPropertyEquals('private_key', 'key');
+    }
+
+    /**
+     * Test the return of set_private_key().
+     *
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_private_key
+     */
+    public function testSetPrivateKeyReturnsSelfReference(): void
+    {
+        $this->assertEquals($this->class, $this->class->set_private_key('key'));
     }
 
     /**
-     * Test the fluid interface of set_auth_token().
+     * Test that set_oauth_token() sets the oauth_token.
      *
-     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_auth_token
+     * @covers Lunr\Vortex\FCM\FCMDispatcher::set_oauth_token
      */
-    public function testSetAuthTokenReturnsSelfReference(): void
+    public function testSetOAuthTokenSetsOAuthToken(): void
     {
-        $this->assertEquals($this->class, $this->class->set_auth_token('auth_token'));
+        $this->class->set_oauth_token('token');
+
+        $this->assertPropertyEquals('oauth_token', 'token');
     }
 
 }
diff --git a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherTest.php b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherTest.php
index 2c63ee9..b22d315 100644
--- a/src/Lunr/Vortex/FCM/Tests/FCMDispatcherTest.php
+++ b/src/Lunr/Vortex/FCM/Tests/FCMDispatcherTest.php
@@ -10,13 +10,17 @@
 
 namespace Lunr\Vortex\FCM\Tests;
 
-use Lunr\Vortex\FCM\FCMDispatcher;
+use Lcobucci\JWT\Builder;
+use Lcobucci\JWT\UnencryptedToken;
 use Lunr\Halo\LunrBaseTest;
+use Lunr\Vortex\FCM\FCMDispatcher;
 use Lunr\Vortex\FCM\FCMPayload;
+use Mockery;
+use Mockery\MockInterface;
+use PHPUnit\Framework\MockObject\MockObject;
 use Psr\Log\LoggerInterface;
 use WpOrg\Requests\Response;
 use WpOrg\Requests\Session;
-use ReflectionClass;
 
 /**
  * This class contains common setup routines, providers
@@ -50,6 +54,18 @@ abstract class FCMDispatcherTest extends LunrBaseTest
      */
     protected $payload;
 
+    /**
+     * Mock instance of the token builder class.
+     * @var MockInterface<Builder>
+     */
+    protected $token_builder;
+
+    /**
+     * Mock instance of the token UnencryptedToken class.
+     * @var MockObject<UnencryptedToken>
+     */
+    protected $token_plain;
+
     /**
      * Instance of the tested class.
      * @var FCMDispatcher
@@ -68,6 +84,10 @@ public function setUp(): void
                               ->disableOriginalConstructor()
                               ->getMock();
 
+        $this->token_builder = Mockery::mock(Builder::class);
+
+        $this->token_plain = $this->getMockBuilder(UnencryptedToken::class)->getMock();
+
         $this->class = new FCMDispatcher($this->http, $this->logger);
 
         parent::baseSetUp($this->class);
@@ -78,6 +98,8 @@ public function setUp(): void
      */
     public function tearDown(): void
     {
+        Mockery::close();
+
         unset($this->logger);
         unset($this->payload);
         unset($this->class);
diff --git a/tests/phpunit.xml b/tests/phpunit.xml
index 59c5fca..d0676d9 100644
--- a/tests/phpunit.xml
+++ b/tests/phpunit.xml
@@ -47,6 +47,8 @@
             <file>../src/Lunr/Vortex/FCM/Tests/FCMResponseBasePushSuccessTest.php</file>
             <file>../src/Lunr/Vortex/FCM/Tests/FCMResponseGetStatusTest.php</file>
             <file>../src/Lunr/Vortex/FCM/Tests/FCMDispatcherBaseTest.php</file>
+            <file>../src/Lunr/Vortex/FCM/Tests/FCMDispatcherConfigureOAuthTokenTest.php</file>
+            <file>../src/Lunr/Vortex/FCM/Tests/FCMDispatcherGetOAuthTokenTest.php</file>
             <file>../src/Lunr/Vortex/FCM/Tests/FCMDispatcherSetTest.php</file>
             <file>../src/Lunr/Vortex/FCM/Tests/FCMDispatcherPushTest.php</file>
         </testsuite>
diff --git a/tests/statics/Vortex/fcm/oauth_error.json b/tests/statics/Vortex/fcm/oauth_error.json
new file mode 100644
index 0000000..e31d06c
--- /dev/null
+++ b/tests/statics/Vortex/fcm/oauth_error.json
@@ -0,0 +1,4 @@
+{
+    "error":"invalid_grant",
+    "error_description":"Invalid JWT: Token must be a short-lived token (60 minutes) and in a reasonable timeframe. Check your iat and exp values in the JWT claim."
+}