Haramasetai, Anta wo - Ω Ochi Gokudou wa Sodateta Bou ni Kamareru

Jin Sayama is the only beta in a yakuza family composed entirely of alphas.
Despite that, he's managed to rise up the ranks and win so much respect, he's been charged with taking care of the boss's second-eldest son, Ryo.

But, one day, in the midst of a turf war, Jin gets splashed with a mysterious substance. At first, he thinks it's just an aphrodisiac... but before he knows it, he's been transformed into an omega!
His body starts burning up and emitting pheromones that signal his presence to alphas. Ryo notices something's wrong and touches him... making Jin's behind start throbbing instinctively for Ryo!
He knows it's wrong, but he lets Ryo ravage him. After a fateful bite on the neck, he's Ryo's forever...

This is an omegaverse story featuring a younger alpha and a stubborn older gangster brought together by irrepressible instinct!

Original Manga
Official English