First, this is pfSense and not m0n0wall
Second, I assume the clients at OPT are using another default gateway that doesn't have a route back to your LAN subnet vie the pfSenses OPT IP. Masquerading would fix that but could cause other trouble on the other hand. Adding a route at the OPT's clients default gateway would be the "cleaner" solution imo. If you reall wan't to NAT enable advanced outbound NAT at Firewall>NAT and add a mapping for LAN to OPT with OPT IP of the pfSense there.