i've been playing with op2 behind cable, routers, whatever for a while, and i've figured out these:
if ur behind a router, it's not possible to host without getting your computer directly connected to the internet, since none of the routers seem to be able to handle some of the messages a op2 host sends to the users
you can join someone else not behind a router or firewall though, but you have to forward 47800, 47777, and/or 47776, TCP and/or UDP to your computer(i set these all while i was trying to figure out the port #s outpost uses, but i'm pretty sure you dont need one or two for it to work)