I already know about OA on Linux, and it's great to be able to play it on the Mac. The key to getting it working in OS X is to grab the v0.6 Linux tarball from the official site (openarena.ws) and decompress it to obtain a directory where you can simply drop the ioquake3 app in. Of course, you can rename the directory to...OpenArena if you like. You can also delete everything inside the directory except for baseoa and the ioquake3 app.

Be sure to read the wiki...

Gameplay is pretty solid for a basic port, although there are a few missing things...none of them showstoppers in any way. The 0.7.0 port is currently in development as of this review.

Did I mention that it is FREE?