There is a variety of thing you can do. But it totally depends on what you are willing & can do.
From a graphical standpoint:
- Many of the maps need a face lift.
- New textures to coincide with the Q3 textures
- New/improved Weapon models
- Fix some bugs in a few game characters
- Create some models for mapping
Coding side:
I'm not fully clued into the coding side of OA atm. I suggest checking out the IRC & talking to Sago007.
Others:
- Audio
- Documentation
- Testing the latest builds & giving bug reports
- And much more that I haven't thought of at this moment.