A wish for cooperative gametypes was stated on oa-forums so I remembered good old HUNT for Q3A.
This mod offers two new gametypes that bring coop experience and serious weapon changes. You shouldn't play this alone as it gets way harder then. One gametype is named Save The Universe which is played in normal FFA/TDM/CTF maps that come with Open Arena. The mission is collecting a certain amount of artifacts that are symbolized by the quad powerup.
The second coop gametype is called Escape From Hell and takes place in long, randomized maps where the goal is simply reaching the exit.
Of course both types of play are not easy as players are not alone in the maps and monsters of various sizes and skills try to stop them. That's where cooperative play comes in. There is a certain amount of lives players share for every map and respawn is only possible if at there is at least one player alive and there are enough lives left.
The mod runs out of the box except the default monster models which are set to Quake 3 player models (klesk, tankjr) and have to be changed. This can be done by HUNT options menu or via console. A small config could do this for you. This config is included in a small additional xoa_textures.pk3 file I created to solve a texture missing problem as one of the randomized maps that comes with HUNT uses Q3A textures.
A playable mod with all required files is downloaded when you connect to the running HUNT server if you enable automatic downloading on your client. If you connect a second time sometimes you get disconnected quickly without entering the game. This happens when connecting from standard Open Arena and can be fixed by launching the mod first (via mod menu) and making sure the right monster models are set (eg. '/exec autoexec.cfg' if you already have xoa_textures.pk3 in your HUNT folder) and then connect to the server.
To have a little coop challenge this server creates a HIGHSCORE of each map played. So the fastest missions and the highest scores appear in these tables. I'm curious who will appear there over time. :)