Are there any actors or actresses that have won an Academy award for best actor or supporting actor for their role in a sci-fi film?
You have to use a fairly inclusive definition of "sci-fi" for a couple of these but...
Fredric March won Best Actor in 1932 for Dr. Jekyll and Mr. Hyde.
Cliff Robertson won Best Actor in 1968 for Charly.
Don Ameche won Best Supporting Actor in 1985 for Cocoon.
Heath Ledger won Best Supporting Actor in 2008 for The Dark Knight.