Yan, by TKO but since it's only 3 rounds and Yan tends to start slow, it could be a decision win. But I do think after the Aljo loss, he's going to start picking up the pace in round 1 and not be a slow started anymore since that cost him his belt.
I also do expect O'Malley to do something funky to try and discredit the win, or to get a disqualification/no-contest. He has a pattern with his losses and fights he was going to lose. So Yan will have to finish him before he gets a chance to do that. Once O'Malley knows he isn't going to win he looks for a way out.