It's worth noting that .ogg is a container format like .avi or .asx, it can contain audio and video streams in various formats. I think the list of supported codecs is (theoretically): Vorbis, Speex (low-bandwidth speech codec), and FLAC (lossless) for audio, and Tarkin (experimental wavelet-based) and Theora (open continuation of On2's VP3 codec) for video. Vorbis+Theora OGG files definitely exist, the rest I'm not sure about.