this was harder to find than I thought.
Not hard, just not trivial; this should be a one-click add-capability type action,
but the instructions were not as clear. (Ideally, iTunes would already have support for open formats.)
- Quit iTunes
- Download XiphQT QuickTime component.
This is a small 1M .dmg; double click to open it. You'll see some text files, but most importantly
- Copy it in to /Library/QuickTime
- [Optional] This is only necessary if you want to double-click an .ogg file and have it open in iTunes. You can still use the 'add to library' menu item or drag files from the Finder on to iTunes. I think.
Edit the info.plist file inside of iTunes;
see instructions here: http://en.wikibooks.org/wiki/Using_iTunes/Ogg_in_iTunes
Start up iTunes and play.
I did this on 10.6.4.