There are packages for installing Ogg codecs on the N800, but the built-in media player won't recognize Oggs when scanning the filesystem... I never got Ogg Vorbis working on the N800, except with mplayer. I'm currently working on a usable frontend for mplayer (for both, video and audio).

The 770's built-in media player can play Ogg Vorbis just fine after installing the codecs, though.