I have a large collection of oggs that work fine in itunes using the plugin from here:
I thought I would try the one here as it seemed to be still in active development.
I wiped the itunes folder and re-registered all of the oggs.
Most of them are detected fine but some have a time of "not available" and are just skipped when attempting to play them.
I don't know what this can be caused by at all. I have changed the tags and the filename.
Try the modified version of this plugin which you can download at http://doublecommand.sourceforge.net/qtoggvorbis.html
I also had some files which wouldn't play with the new version of this plugin (apparently the issue arises with files created/modified by OggDrop), but Michael Baltaks' patched plugin plays them all right.