Question

Which MIB do I need if I am trying to find the routing table of a Linksys WRT54G with openWRT installed on it?

Answer 1

That should be MIB II. The SNMP ObjectID (OID) is .1.3.6.1.2.1.4.21 which translates to ip.ipRouteTable. (This works for me on Windows, so I see no reason why it should be different for your Linksys/openWRT system.)

Here is the hierarchy table for the routing table leaf:

1 - ISO assigned OIDs
1.3 - ISO Identified Organization 
1.3.6 - US Department of Defense 
1.3.6.1 - OID assignments from 1.3.6.1 - Internet 
1.3.6.1.2 - IETF Management 
1.3.6.1.2.1 - SNMP MIB-2 
1.3.6.1.2.1.4 - ip 
1.3.6.1.2.1.4.21 - ipRouteTable