No. Oswalt is one of the most gifted young starters in the game, up there with guys like Prior, Beckett, Webb, Oakland's big three, etc. Anybody who knows their pitching will tell you that he's clearly the ace of this staff. Pettite is past his prime. Clemens is way, way past his prime, although as the Cheat pointed out, he did well on the road last year.
But then again, there are plenty of intelligent baseball people out there who would go with Pettite as the ace. To me that's just dead wrong. But with enough run support, he could still win 20. Oswalt, however, doesn't need run support to win 20. That's the difference.