He took a dive against Hopkins.
As much as I like Oscar, I have my doubts about that one too. Sure, the punch looked right on target but I am not so sure about his reaction. But then again, why would he take a dive? I don't see how that would benefit him.
He is a cross dresser as pointed out above.
If those pictures were really him, is it really our business to see what he does in private? Look at the girl he was with, I am sure many guys wouldn't mind wearing a tutu just to be with a girl like her! Girl looked quite sexy and in great shape.
He recieves gift decisions. Again as pointed out.
As well as gift decisions to his opponent: Trinidad, Mosley II, among others.
He now owns The Ring magazine which gives us further suspicion that he will invade further with fake drama.
I would agree with you if I saw Hopkins win the last fight just because he was under GBP. But nope, he lost fair square. Besides, we see bad decisions all the time only that ODLH is not involved so people find those bad decisions ok.

These are all very valid reasons why he isn't very many peoples favorite fighter.
Also, because he has beaten many fighters who are fan favorites, look at his resume. I didn't like him beating JC Chavez and Rafael Ruelas among others on his list!