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Hi, I just want to re-emphasis what Rolf already described. The OSPF network statement doesn't directly advertise a network, it "looks" for "up" interfaces that have an IP that matches against the network statement (als also mentioned by Rolf, somewhat like an ACL). Once if finds such an interface, that interface will be included in the OSPF. As Rolf also notes (by default) active OSPF interface are what are used to form adjacencies with neighboring OSPF routers.
Also, when it comes to what network's mask is, that is determined by the interface's IP mask.
The area parameter determine which OSPF area the interface will be part of.
Something else to note, the network statements can overlap in matching, allowing most of an address block to be assigned to one OSPF area, while allowing more specific subnets, within the address block, to be assigned to another OSPF area.
Usually, you'll see network statements that match specific IPs or you'll see a single network statement that matches all (or many) interfaces IPs.
e.g.
int x
ip address 192.168.1.1 255.255.255.0
int y
ip address 192.168.2.1 255.255.255.0
router ospf #
network 192.168.1.1 0.0.0.0 area #
network 192.168.2.1 0.0.0.0 area #
or
router ospf #
network 192.168.0.0 0.0.255.255 area #
