beware Cisco IOS newer than 12.2

Very limited functionality with modern IOS-es, and typical MPLS VPNs. It is a IOS limitation, not the plugin itself. The only chance is to configure ospf traps on the router and trap handling, passive service on Nagios. IOS config:
snmp-server host ….
snmp-server enable traps ospf

“Cisco IOS Release 12.0(26)S, 12.2(25)S, 12.2(27)SBC, 12.2(31)SB2 and later
releases, the OSPF MIB and CISCO OSPF MIB will be supported only for the first OSPF process [….] SNMP traps will be generated for OSPF events that are related to any of the OSPF processes.
There is no workaround for this situation.”

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/iro-mib-1850.pdf