Mosley has beaten De la Hoya twice already - once as an amateur and another as a professional. It looks to me like Shane has Oscar's number. I wouldn't be surprised if Mosley wins again.
There seems to be a trend going on that if a boxer loses an amateur fight against another boxer, he'll lose again if they meet on a professional fight. An example would be Vernon Forrest and Shane Mosley. Forrest beat Mosley as an amateur, and beat him again twice as a pro. I believe there is another well-known boxer with the same dilemma, and Oscar might be another one.
Nonetheless, on recent interviews Mosley has appeared a bit overconfident. That may prove to be his downfall against De la Hoya. De la Hoya admits he made a mistake against Mosley in their last fight, and promises he won't be the same boxer that Mosley met 3 years ago.