OSPF: Why we have to remove all network commands before ip ospf area in interface node

[pnxtm]

Instead of the OSPF node, we can enter our networks through an interface node with the command ip ospf area, but it seems we can't mix that. Either we specify all networks in OSPF node or we specify them in its interfaces.

We have warning in both side:

Interface node:

deb(config-if)# ip ospf area 1
Please remove all network commands first.

OSPF node:

deb(config-router)# network 172.40.0.1 0.0.255.255 a 2
Please remove all ip ospf area x.x.x.x commands first.

Why can't we define some of our networks in the interfaces and others through the OSPF node and network command ?