How can I monitor the ospf routing table change
This all depends a little bit... but after using Google and some testing against a Cisco device, I found out this:
1 - ISO assigned OIDs
1.3 - ISO Identified Organization
1.3.6 - US Department of Defense
22.214.171.124 - OID assignments from 126.96.36.199 - Internet
188.8.131.52.2 - IETF Management
184.108.40.206.2.1 - SNMP MIB-2
220.127.116.11.2.1.4 - ip
18.104.22.168.22.214.171.124 - ipRouteTable
what did not help me - not even with a WALK - so I kicked out .21 - using this OID:
and did a walk - and there I could see all my routes...
Now - you asked specifically for the routing protocol OSPF - what adds a lot more complexity to this task - and I don't have it enabled and building a test-network with it would honestly take to much time for me personally, but here is a Cisco article about it: https://www.cisco.com/c/en/us/support/docs/availability/high-availability/15408-ospf-snmp-config.html
Interesting or a something to start with might be any Error/Status change counter in the Interface or the Neighbors table...
Probably the LSA count as well..
But I admit that it is a while that I used OSPF - I would need to review how it worked in detail before I would be able to monitor it - but from what I see there and assuming you have the knowledge and protocol specifications in mind (unlike me who forgot half of it or more...) you should be able to figure this out in no time and create some fancy SNMP sensors.
You did not specify if you use a Cisco router / switch at all - so I am not sure if either of those answers will help you - but yes it is possibly assuming your vendor supports it.
Don't have one - so I am kinda out - but PRTG support might be able to help out.. there was a blog from PRTG about monitoring Fortigate firewalls lately, this might be a good start.. but I am out in this case..
Sorry for that - can't simulate it... :-(
Your best bet would be to start with the FORTINET-FORTIGATE-MIB.mib file. This is most likely available through a support portal. Once you have that, import it with our MIB Importer, and see if it has OIDs for OSPF updates.
If not, you would be best to contact Fortinet to see if they have the capability to monitor OSPF changes through SNMP with their FortiGate products.