You never heard much about Oscar's chin, It was under the shadow of his devstating left hook. Then people somehow labeled him as this pure boxer, and I don't know where the hell that came from.
The only time his was ever questioned was when he at the lower weights, after he suffered a couple of flash knockdowns to Campanella and Valenzuela. That carried over when he hit 140-147, I remember reading many comments on how Trinidad and Quartey would destroy him. After his fights with them, the questions about his chin disappeared and he is now considered to have had one of the great chins, given how many top fighters and punchers he fought and was never in serious trouble.
Oscar had a great chin. Look at his resume, many heavy hitters, accurate punchers, speed demons. Been to the canvas a couple times, mostly flash knockdowns in his earlier fights. Even after the body punch KO by Hopkins, he was fine and back to normal.