The following guide will help you to play Ogg Vorbis files from iTunes. This guide will work for both Macintosh OS X or Windows.
First, you must download an Ogg vorbis plugin for quicktime. iTunes has support for QuickTime formats, so adding the ability to QuickTime to play Ogg files indirectly also allows iTunes to play them. You can get a good plugin for Ogg from the Xiph project.
After downloading and installing the codecs, you may need to restart your computer. Next, we just add some Ogg files to iTunes and try it out. One thing you'll notice is that metadata for the Ogg file is not imported. This behavior is similar to how wav files work in iTunes. It is a product of how iTunes deals with some QuickTime formats. You can read more about this issue in this xiph metadata support ticket.