iTunes - Play .ogg vorbis files in iTunes

By default, you cannot play .ogg files in iTunes, as they currently are not supported. iTunes uses Quicktime libraries in the backend to handle playback of audio and video content. We can, however, use a utility to allow Quicktime to handle files of different types.

Xiph
Xiph QuickTime Components (XiphQT) is, in short, the solution for Mac and Windows users who want to use Xiph formats in any QuickTime-based application, e.g. playing Ogg Vorbis in iTunes or producing Ogg Theora with iMovie.

Download
http://www.xiph.org/quicktime/download.html

Bonus
Also: How to add FLAC files into iTunes