idk the specifics of omegaverse and stuff but here is one manga ive read where the alpha is topped by the omega
http://www.mangago.zone/read-manga/a_tough_alpha_wants_to_be_loved_cornered_by_a_young_omega/
Not impossible but it is rare. Lets say Alpha-Omega pairing is the same as all the straight couple. They are the majority but theres also gay, lesbian, bisexual, trans people and couple right? its kinda like that.
But most importantly it all depends on author and the ABO world they are creating.
After all other than the gender, pheromones and pairing system there is no other rules that one must follow when creating an ABO story or world (even then they can still play and put their own twist on those tropes/rules if author wants to)
For example:
in "Love is an illusion- http://www.mangago.zone/read-manga/love_is_an_illusion/?c8kF " Fargo made a different type of bonding/pairing system where Alpha doesn have to bite the nape of omega to become a pair.
"Kashikomarimashita Destiny - http://www.mangago.zone/read-manga/kashikomarimashita_destiny/ " is a manga where everyone can become pregnant be it Alpha male or Beta Male which is rare to see in most ABO story.
In this manga http://www.mangago.zone/read-manga/falling_alpha_enigma_the_fourth_bath/ author invented a new type of gender called Enigma that is superior to even Alphas.
Omegas can top alpha as well if they are dominant and are willing. Heres 2 amazing story with omega toping Alpha
http://www.mangago.zone/read-manga/boku_no_high_spec_kareshi_sama/
http://www.mangago.zone/read-manga/like_a_tidal_wave_from_pheromone_shower/
So yea.. not impossible. If there is not diversity then ABO genre would be so boring.
I'm a newbie in the omegaverse, so here's a question for you experts: can an alpha be a uke/ an omega a seme? Or is it impossible because of some hormonal reason i don't get yet ? (if it is i'd love to see that (✧∀✧)/)