Mayorga can be dangerous. He's absolutely fearless in the ring and has the ability to land right hand power shots from multiple angles. If De La Hoya tries to go toe-to-toe, then Mayorga might win. Every big puncher has a chance to win from time to time, even against a superior opponent.
However, if Oscar _boxes_ like Spinks and Trinidad did against Mayorga, then Oscar will definitely win a decision, maybe even a TKO. Oscar is a better boxer, a more experienced boxer, and (imho) a better trained boxer. If he boxes and doesn't give any rounds away like he did with Tito, the Oscar will have the best chance to win.
It does look like Mayorga has seriously wounded Oscar's pride with his press conference insults, so Oscar might feel that he has to stand a trade a little bit.
Either way, it should be a good fight.
.. and I'm betting on Oscar.