Holy crap.
All right, so Open Arena is not a mod, it's a stand alone game, based on Quake III Arena.
Quake III and Trem are both stand alone games. So a mod for one game probably won't work for the other game.
If you want to get a Tremulous mod on your server, then just get the mod's
game.qvm file with a matching
tremded.exe file and set up the rest of the files like you would a regular server. For more on how to set up a server: look
here.
P.S. You can only have one mod on a server at a time, unless you either set up a new server with a different mod, or you change the above mentioned files on your current server and then restart it.