Hello Cameron
The enabling of the feature can either be done globally (per area on a router) or on a per-interface basis. Per interface, the command is:
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip ospf authentication
Globally, the command is:
R1(config)#router ospf 1
R1(config-router)#area 0 authentication
Now the configuration of the authentication key must be done on a per-interface basis. This is because you may have a different keys for different neighbors that are connected to different interfaces. This command can only be applied to the interface like so:
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip ospf authentication-key MYPASS
I hope this has been helpful!
Laz