Here's the information on the required MD3 exporter for Blender: http://openarena.wikia.com/wiki/DeveloperFAQ#Modeling
The quake 3 engine requires player models to be separated into 3 different areas, l_legs, u_torso, h_head. These are connected together using "tags" (or empties in Blender). You'll also have to setup a animation.cfg to see your animations in game.
I might write up a tutorial on setting up player models to use with OpenArena.