There is a very interesting list at IMDB, their highlighted list, that is the top 20 most surprising Oscar winners since 1980, posted by Mark Englehart. Mark has done a pretty good job, I agree with many of them.

He has The Pianist with two surprises, Adrien Brody for actor and Roman Polanski for director. I'm sure most felt that Polanski had been slighted for Chinatown in 1974.

At least two