diff --git a/ocaml/xapi/xapi.ml b/ocaml/xapi/xapi.ml
index ed6323663e3..ca87e740efb 100644
--- a/ocaml/xapi/xapi.ml
+++ b/ocaml/xapi/xapi.ml
@@ -180,7 +180,7 @@ let init_args () =
     Xapi_globs.xenopsd_queues := ["xenopsd"]
   )
 
-let wait_to_die () =
+let wait_to_die () : unit =
   (* don't call Thread.join cos this interacts strangely with OCAML runtime and stops
      the OCAML-level signal handlers ever getting called... Thread.delay is fine tho' *)
   while true do
diff --git a/ocaml/xapi/xapi_ha.ml b/ocaml/xapi/xapi_ha.ml
index ddfbc357fb2..b6ba195f823 100644
--- a/ocaml/xapi/xapi_ha.ml
+++ b/ocaml/xapi/xapi_ha.ml
@@ -130,7 +130,7 @@ let uuid_of_host_address address =
 let on_master_failure () =
   (* The plan is: keep asking if I should be the master. If I'm rejected then query the
      live set and see if someone else has been marked as master, if so become a slave of them. *)
-  let become_master () =
+  let become_master () : unit =
     info "This node will become the master" ;
     Xapi_pool_transition.become_master () ;
     info "Waiting for server restart" ;
diff --git a/ocaml/xenopsd/cli/xn.ml b/ocaml/xenopsd/cli/xn.ml
index 4c1251cccbd..a6ed6a884bd 100644
--- a/ocaml/xenopsd/cli/xn.ml
+++ b/ocaml/xenopsd/cli/xn.ml
@@ -1051,7 +1051,7 @@ let unix_proxy path =
   | 0 ->
       let buf = Bytes.make 16384 '\000' in
       let accept, _ = Unix.accept listen in
-      let copy a b =
+      let copy a b : unit =
         while true do
           let n = Unix.read a buf 0 (Bytes.length buf) in
           if n = 0 then exit 0 ;