Hi sachin
OSPF as with other components, will dynamically put routes on the routing table of the PIX. You can even inject default routes on the pix, when using ospf. this will be really helpful, if there are lots of site having static routes everywhere. especially when there are lots of VPN sites connecting to each other and the routes need to be dynamically chagned in PIX.
another thing is, on the PIX you cannot add higher metric routes, which means there can be no backup. If you use OSPF, u can manipulate the costs, and automatically shift the traffic to backup router.
hope this helps.. all the best..
Raj