- Go to the QT Components SourceForge page and download the plugin (there are versions for Windows and Mac
- Inside the disk image you'll find a readme file and OggVorbis.component. Put OggVorbis.component in /Library/QuickTime (as described in the readme)
- Restart iTunes and it will work!
A colleague brought in a load of ogg files. Could he play them under iTunes? No problem! Here's how: