Couple of reasons, Oscar had an amazing chin.
Also, he was hit with some big right hands, especially the one at 2:26 or so in the round, it was a clean, flush right hand that would have KO'd anyone. He also landed some huge left hooks back that Mosley absorbed.
I think the second reason is because while he did get hit with some big shots, his mobility and boxing ability also made most of them land deflected off his gloves or while he was moving.
Marg was hit with so many flush shots, it was ridiculous. Oscar didn't take 20% of those shots in this fight. Marg was basically standing there getting crushed over and over.