Let’s say you have a router with the following interfaces:
For RIP, in order to include all of the subnets of these interfaces in the routing exchanges, you would have to issue the following commands (remember that RIP assumes the classful address range):
This would include all of the networks of all of the interfaces, because for the first two /24 is assumed, for the third /16 is assumed, and for the fourth /8 is assumed, all of which encompass all of the networks.
You could even issue the following command for example:
This would be accepted but would actually be translated to the classful network address like so in the configuration:
And remember, auto-summary on the advertising router (by default) simply allows for the grouping of the networks of these interfaces within the same classful network into single entries in the routing tables of the RIP neighbors.
Now OSPF simply allows you to more specifically state which networks you want to participate in the routing exchanges. For the same set of interfaces, you can have the following commands for OSPF:
R1(config-router)#network 192.168.0.0 0.0.0.63 area 0
R1(config-router)#network 192.168.1.0 0.0.0.255 area 0
R1(config-router)#network 172.16.15.0 0.0.0.255 area 0
R1(config-router)#network 172.16.16.0 0.0.1.255 area 0
R1(config-router)#network 10.10.10.0 0.0.0.255 area 0
R1(config-router)#network 10.20.20.0 0.0.0.255 area 0
Each of the above statements corresponds exactly to each of the networks on the interfaces, including the subnet masks. This is an explicit description. However, we could have also done this:
R1(config-router)#network 192.168.0.0 0.0.1.255 area 0
R1(config-router)#network 172.16.0.0 0.0.255.255 area 0
R1(config-router)#network 10.0.0.0 0.255.255.255 area 0
This too includes all networks on all interfaces.
But OSPF, because it is separated into areas, needs the granularity that the specific subnet masks provide in order to more explicitly state which networks (and thus interfaces) should be advertised and to which area.
Now concerning your question:
There is only one interface that corresponds with the 192.168.12.0/24 network on R1 and that is interface Fa0/0. So if you use a wildcard mask of 0.0.0.255 you are stating exactly that you want this subnet of that interface to participate in OSPF.
I hope this has been helpful!