In XOK, you want to connect ten of your fish and sharks, but your opponent has sharks as well and will use them to take out your fish.
Each turn, you either place two fish on the board on adjacent empty spaces or place one shark.
Sharks come in two sizes, small and large, and you can place a shark either directly on an opponent's fish or adjacent to one or more of your opponent's fish. When you place a small shark, it eats the fish in front of its mouth; when you place a large shark, it eats each fish in front of its two mouths. (Yes, two!) In addition, if you place a shark on a fish, it eats that fish in addition to 1-2 adjacent fish. A shark must eat at least one fish, and all eaten fish are returned to their owner and can be placed again later.
Whoever first groups ten of their fish and sharks wins. If a player cannot take a turn, the game ends, and whoever has the largest group wins, with the number of sharks in those groups being the tiebreaker.