You'll find what you need here:
http://www.cisco.com/warp/public/707/gre_ipsec_ospf.html
and here:
http://www.cisco.com/warp/public/707/tunnel_pix.shtml.
They describe OSPF and multicast in general using a GRE tunnel through Pix firewalls. OSPF uses multicast to establish adjacencies and link state updates.
As mentioned by others, Pix 6.3 due out in April will support OSPF and multicast on its own.
Of course, routing protocols running through and on your firewall have serious security implications. Implement cautiously and use MD5 authentication.
