Here is the two line method to convert a youtube video to oggvorbis video.
Locate clive and ffmpeg2theora in your package and install
$clive <a href="http://in.youtube.com/watch?v=6JeZ5oeAEyU">http://in.youtube.com/watch?v=6JeZ5oeAEyU </a>(replace this with the youtube address you want)
It will create a flv file.
Convert to mpeg video file
$ffmpeg -i AmericaAmerica.flv AmericaAmerica.mpg
Convert to ogg video file
$ffmpeg2theora AmericaAmerica.mpg (replace it with the name of the flv file the previous command created)
Done. You can see the .ogg file in the directory from where you executed the above commands