I know there's already been discussion about Ernie Els. As noted he was the last one to win the Open at Oakmont (1994). Still, this tournament is always full of surprises and you never know when the right player gets hot. Els is 28th on the money list (as of 5/6/07) this year and he did finish 3rd last year at the British Open, so I think he's got a chance at this.
That's why the Open is probably my favorite tournament - the best compete, there's a level of intensity that seems unmatched by any other tournament and sometimes the unexpected happens.