As omega rares are not on the list. the info is this, from Seven. Please correct me if this isn't correct Seven.
there is a 1 in 6 chance (17%) to catch an omega rare instead of a super ultra rare, after you hit L40. So 17% of 2% of 65%, pretty low
I plan on making any omega rare custom a 100% chance due to how rare you catch an omega rare.