Play Othello (Reversi) Online vs the Computer

Play the classic game of Othello — also known as Reversi — directly against the computer, free, in your browser, with no download and no sign-up. You play the black discs and move first; the AI plays white. Click one of the highlighted squares to place a disc and flip the white discs you trap. This is a single-player game built around an honest AI opponent: Beginner makes mistakes you can punish, Medium plays a solid positional game, and Hard runs an extremely strong search engine that thinks about a second per move. Legal moves are shown for you, the live disc count is always on screen, and your wins, losses and draws are tracked per difficulty.

How the AI thinks

The opponent here is not a set of canned tricks — it is a real game-tree search engine, the same family of algorithm used by classic chess and Othello programs. On every turn it generates the legal flipping moves, then runs negamax with alpha-beta pruning: it plays out possible futures for both sides, scores the resulting positions, and picks the move with the best guaranteed result, skipping branches that provably cannot matter. The scoring is what makes Othello hard: rather than just counting discs — which flip constantly and mislead beginners — it uses positional square weights (corners are gold, the squares next to them are traps), mobility (how many moves each side has), and, as the board fills, it reads the game to the end and counts the exact final discs. Hard uses iterative deepening within a time budget of about a second; the whole engine runs locally in your browser (in a background worker thread, so the page stays responsive), and nothing is sent to a server. It is extremely strong against human play, though we do not claim it is perfect.

How to play Othello — the rules

• Start from the centre. The board opens with four discs in the middle, two black and two white on a diagonal. Black moves first.
• Flank to flip. Place a disc on a highlighted square so that it traps a straight line — horizontal, vertical or diagonal — of the opponent's discs between your new disc and another of your own. Every trapped disc flips to your colour.
• You must flip. A move is only legal if it flips at least one disc. If you have no legal move you pass, and the game tells you when that happens.
• Most discs wins. When neither side can move — usually when the board is full — the discs are counted and whoever has more wins. Equal counts are a draw.
• Hint and undo. Hint asks the engine for a strong move; Undo takes back your last move and the AI's reply.

Tips to beat the AI

• Take corners, avoid the squares next to them. A corner disc can never be flipped and anchors whole edges. But playing the square diagonally next to an empty corner (an "X-square") usually hands the corner to your opponent.
• Don't grab discs early. The player with fewer discs in the midgame often has more mobility and better moves. Flipping everything in sight early is a classic beginner trap.
• Limit your opponent's moves. Aim for positions where the AI has few legal moves — ideally forcing it to pass, which lets you play twice in a row.
• Think about the edges. Stable edge discs that can't be flipped are almost as valuable as corners; unstable edge discs can be turned against you.

FAQ

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