idk, i use oggs all the time when i convert a lossless file to a format osu likes. i'd expect to see oggs at least occasionally but every time i dig into a map's assets, it always has an mp3. mp3 is outdated and relatively unifficient. oggs give you better quality for the same filesize. it's even more important when you consider that ranked maps need a bitrate of 192 kbps or less. at that small of a bitrate oggs sound better. period. the best part is that you could go down to like 128 kbps and still retain good audio quality. mp3s at 128 kbps sound like crap. oggs don't.