First, let me state that I have zero experience with the Omni. But I am also of firm believer in RTFM, even for stuff I'm only dreaming about.
Unfortunately, the documentation of the Hardware Setup is a little bit confusing.
In the discussion of the Monitoring tab beginning on page 26, you'll find the following paragraph on page 27:
Changes to the Monitor page are reflected in the
Output options on the Main page in the Hardware
Setup dialog, the Output and Bus pages of
the I/O Setup dialog, and in the Output path
names for tracks in your Pro Tools session. Because
of this, you should always configure the
Monitor page first, before configuring the other
pages of the Hardware Setup and the I/O Setup.
(Emphasis added)
If you did not follow this procedure, it might be a good idea to first go to each of the other tabs and click the "Set To Default" button (and you have to do this on each tab.
Do the "Monitor" tab setup as outlined in the manual, then do the rest of the Hardware setup.
And if you did do things in accordance with the manual, allow me to quote SNLs esteemed Emily Litella..."Never mind"...