I recommend that you carefully evaluate your need for OSPF through a firewall, and see if there isn't another option. It's not that it can't be done. It can, but you create unnecessary security risks by doing so.
The first question I would ask is this: If you don't trust the people on the other side of your firewall, why are you trusting the routing advertisements they send you? They could advertise incorrect routes and bring down your network. It's a powerful denial-of-service attack.
In order to let OSPF through the PIX, you have to create a GRE tunnel through it and run OSPF through the tunnel. I think this is a pretty big hole through the PIX.
Another option is to run BGP across the PIX and redistribute on both ends. This lets you control what routes you advertise, and more importantly, what routes you accept. You can filter so that you don't accept routing advertisements for networks on your side of the PIX, nor advertise networks that don't belong to you.
Another advantage is that you only have to open one TCP port for BGP and then only to the peer addresses -- a relatively small hole.
