By default, the Router ID of an OSPF enabled router is set by the highest address of any online loopback interfaces, or failing that the highest address of any up physical interfaces.
The router ID is selected when the OSPF process initialises, and does not change until the the process has been restarted.
The entire topology is built upon a OSPF router ID, setting a static RID will help keep the network predictable and easily identifiable.
The router ID can be configured to any unique four octets in length, but generally does represent an IP address that is configured on the router.
To set the router ID for an OSPF process to 126.96.36.199:
router ospf 1 router-id 188.8.131.52
After changing the router ID/RID, the OSPF process can be restarted in privileged mode:
clear ip ospf process 1