Tic Tac Toe Online description

Rules.
1. Each turn, you mark one of the small squares.
2. When you get three in a row on a small board, you’ve won that board.
3. To win the game, you need to win three small boards in a row.

You don’t get to pick which of the nine boards to play on.
That’s determined by your opponent’s previous move.
Whichever square he picks, that’s the board you must
play in next. (And whichever square you pick will determine
which board he plays on next.)

A few clarifying rules are necessary:
1. What if my opponent sends me to a board that’s already been won?
In that case, congratulations - you get to go anywhere you like,
on any of the other boards.
2. What if one of the small boards results in a tie?
That the board counts for neither X nor O.