Wrong strategy.. Fighting while giving away your biggest advantage. And Pac's gameplan. I dont think he starved, he was 154 when they signed the deal, and has been at mid 140's for pretty much his training days. He felt good coming into the fight, then said he cant do it anymore when it ended.
def the weight drain. he shouldve allowed himself to rehydrate up to around 165 where he would be at his more natural weight.
Its a combination of wrong strategy, over confidence, and Pacquiao's speed, and skills. If he was weight drained he still should have been able to do something early in the fight. I think he thought Pac was going to rush him right away, and during the brawl he would catch him with his hook. But Manny was boxing moving in and out, and landing solid unanswered blows. Oscar did not know what to do, and his corner didn't have any answers for Pacs strategy. As each round went by more doubt crept in Oscars head, and the less he wanted to be in the fight.
I think weight drain did have something to do with it although Gatekeeper brings up a good point that even if he was weight drained he should have been able to do something early which he never did. So you cant take anything away from Pacmans performance and greatness. However I do think bad strategy and weight drain were both in affect and just added to Oscars defeat. I attribute most of it however to Pacmans strength, speed, and heart.