oggzrip -c theora input.ogv > output.ogv
Likewise, to extract the audio track from an Ogg video:
oggzrip -c vorbis input.ogv > output.ogg
Ogg is a container format, which means that it can contain several files of different types and play them at the same time. Vorbis is an audio format and Theora is a video format, so when you watch an Ogg video, it is playing back the Theora movie track at the same time as the Vorbis audio track.
The oggzrip command can be used to extract single tracks from the Ogg container. The "-c" option allows you to choose which track to extract. In the first example, "theora" is the chosen track, so only the video (without sound) is extracted and output to the file "output.ogv". In the second example, "vorbis" is the chosen track, so only the audio is extracted.
These instructions were painfully arrived at on Ubuntu Jaunty (9.04). See the oggzrip man page for more information.