I'm assuming you just want to be able to play it on the GP2X?
Just rename it to AVI. The support is there, it's just the extension that it doesn't look for.
DivX and Xvid are just video formats OGM is a container which bundles DivX/Xvid video with a sound file.
If you want to extract the video from the OGM container you can just use VirtualDub