http://scratchpad.wikia.com/wiki/OpenArena/serversThis is a useful link on how to set up your sever...though it does seem to be something for advanced users only. Using this outline requires knowledge on how to use cfg files for Quake 3.
Another method is by opening Open Arena on the computer intended for the server, set up the match options, set the dedicated mode on internet, and launch the server. Once that's done, go to the computer you intend to play on, find your server (or input the IP address), connect, join, and play. Note that you don't have to open the server on a separate computer (you can run the server and play on the server on the same computer), though it's recommended that you set it on a separate computer so the computer you play on doesn't take a beating from the excess ram (OA's dedicated server doesn't take much memory to run, but when running bigger mods like True Combat, Urban Terror, Afterwards, etc. etc., it will take much more memory that you will want to run the server separate from the client)