What animals prey on the orca whale?

Nothing eats the orca whale besides, big Great White Sharks and humans
Young Killer Whales are theoretically hunted by large sharks, although this is considered unlikely and has never been witnessed. As adults, it is theorized that Great White Sharks may try to "sneak-attack" stray adult specimens. This, again, is theoretical, as it has never been witnessed, and Killer Whale almost never 'stray' from their family pods. Killer Whales are also far larger then even a very large Great White's normal prey size.