I have a customer who is adding a second site, all calls to come in to the OS7200S at the first site via PRI. We're going to network the sites together via SPNet, just wondering if we can send fax calls over SPNet to the second site.

Both sites use OAS cards, V4.75 software.

Yes, the OAS cards support Fax over IP.
How well it will work is another matter.
Fax over IP is not reliable