Responses
Ayashi no hakoniwa ni ukabu tsuki...
not really omegaverse but this sounds exactly like you're describing. maybe this is it?
https://www.mangago.me/read-manga/ayashi_no_hakoniwa_ni_ukabu_tsuki/
Hey guys! Looking for this japanese omegaverse manga. Bottom is an omega teacher, he teaches in a monster school. The top's monster transformation is a big spider