Here is some info from OA.ws forums: command for record a demo:
/record <filename>
/autorecord (if u are on cpma or osp)
/g_synchronousclients 0 (so wont lag as hell)
/stoprecord (to stop it)
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To record you can turn the settings down a bit in fraps to record videos that arent nearly as huge. it also helps if you use a lower resolution (640x480 works nicely) when recording videos if you are recording them at full size in the fraps settings. changing the fraps recording framerate to 25fps helps a bit as well.
I would also suggest recording with xfire instead (edit quake 3 arena's entry in xfire to work with open arena or make a custom config), but in my experience for some reason it makes all of my recordings really dark.
Useful commands:
/set timescale 0.1
/video
/demo [your-demo]
Path:
in windows C:/Documents and Settings\yourusername\Application Data\OpenArena
always saved as: <filename>.dm_68