Anything that gives even 1 XP has a chance to drop an omnibit. It's the same chance whether you get 1 XP or 696969 XP. NO MORE AND NO LESS THAN 1 OMNIBIT (2 if you have the cosmic rewards perq).

So it stands to reason that if you drop down to the lowest level zone where killing a mob gives XP, , that would max out your omnibit rate. I'd say there's probably a drop cooldown preventing you from actually farming them. Just play the game, don't afk, and they will come in slowly but surely.