it's hard to tell who wrote the first omegaverse because omegaverse as we know it was cobbled together by multiple fanfiction authors (from different fandoms, including supernatural, star trek etc) drawing inspiration from one another and building on each other's ideas with slight differences.
Omegaverse as we know it came from the Supernatural fandom. Which tracks with the fact that Dark Angel, a tv show Jensen Ackles (one of the Supernatural main characters) starred in, also featured heat. Fans likely drew inspiration for the concept of heat cycles in omegaverse from this tv show (or maybe from star trek. who knows)
And then omegaverse got popular in china and japan which led to more original works featuring omegaverse, instead of being featured mostly in fanfiction
I read my first abo 4 years ago not knowing it was an abo. Got me wondering who wrote the first abo?
Aka omegaverse.
Abo - alpha, beta, omega (for those who might be new to it)