Hi,
Maybe the easiest way to do this is using ffmpeg.
You can install it on Ubuntu by typing the following on the command line;
sudo apt-get install ffmpeg
Once installed, cd into the directory where your .ogg file is and type the following to convert it…
ffmpeg -i myvideo.ogg -b 384000 -s 610x400 -pass 1 -passlogfile log-file myvideo.flv
In the above example I’ve converted it into FLV, so displaying on the web with flash, you could, of course, change the extension of myvideo to avi or whatever you need.
Check this links for more…
http://ffmpeg.mplayerhq.hu/ffmpeg-doc.html
http://www.smorgasbord.net/converting-video-in-linux-using-ffmpeg-and-mencoder/
Hope that helps!