Probably you've seen, when you upload a MP3 file or other audio files, that their download appearance turns into an audio player.
Now for .ogg it does have an audio player. But it doesn't have one for .oga.
I would consider this feedback as low priority, depending on how hard it is to add it.
But, you can simply rename .oga to .ogg and then the audio player will show when you upload it.