I think every male Omega is instinctively an uke because of their biological make up now I'm not 100% sure of this, but that makes the most sense in my opinion
there is one manga called High-Specs Boyfriend where the Omega wants to be a seme and finds himself an uke alpha. But for the most part they treat omegas like women concerning sex (they are there for the penetrative pleasure)
Sutenaide My Hero
it's an OmegaxAlpha
http://www.mangago.me/read-manga/sutenaide_my_hero
It's not completely translated in English. But you can find the raws and in Spanish
Wondering this only now, but so practically every omega is by instinct an uke?? Or can they be mates but the omega is the seme?