What is Omok?
Omok is five in a row as it is played in Korea, where the name simply means "five stones". It is played on a 19×19 board, the same board as Go, and a line of five or more stones of your colour wins, so six in a row counts.
The special rule is the ban on the double three, sometimes called three and three. An open three is a line of three stones that can become a four open at both ends with one more stone. Neither player may place a stone that makes two open threes at once, because such a move usually wins by force. A double four and a four with a three are both allowed, and a stone that makes five is always allowed, even if it also makes threes.
The game will not let you play a forbidden point. Unlike Renju, the restriction applies to both players equally. If every empty point is forbidden to the player who must move, the game is a draw, and so is a full board with no winner.
Rule changes
| Rule | Standard Gomoku | Omok |
|---|---|---|
| Board | Standard Gomoku15×15 points. | Omok19×19 points by default, the size of a Go board. 13×13 and 15×15 can be chosen in the setup. |
| How to win | Standard GomokuMake an unbroken line of exactly five stones. | OmokMake an unbroken line of five or more stones. |
| Double three | Standard GomokuAllowed, and often a winning attack. | OmokForbidden for both players: no stone may make two open threes at once, unless it also makes five. |
| Double four and four-three | Standard GomokuAllowed. | OmokStill allowed. Only the double three is banned. |
| Draws | Standard GomokuThe board fills up with no winning line. | OmokA full board, or a position where every empty point is forbidden to the player to move. |
History
In Korea, five in a row is known as omok and has long been played on the Go board. It is one of the most familiar board games in the country, played at home, at school and online.
The ban on the double three is the rule that sets the Korean game apart. It has the same purpose as the much stricter restrictions on Black in Japanese Renju, taking away the easiest winning attack, but Omok applies it to both players and leaves fours alone. Local rules differ in details, and the version here follows the most widely described form: five or more wins and the double three is forbidden for everyone.