Is it possible to play Open RA over a LAN network? I couldn't figure out how to make it work and suspected that it's not possible because you need to forward the ports.
Yes, you can play over lan, but the server browser will not work.
If you know the ip and port of the host, you can connect directly to it over lan using the Direct Connect feature.
You don't need to forward any ports if all the machines are in the same lan.