If you are playing a game of Tic Tac Toe, the first player will likely play in the center. If you are "O", you have two choices: either you play in a corner or in the middle of one of the sides. If you play in a corner, it does not matter which corner you choose because the whole board could be rotated. Similarly if you play in a side. If you are "O", you should play in a corner. This is because you can guarantee that you will at least tie the game, regardless of the strategy of "X". If you play in a side, then "X" can win if he/she plays well enough. For fun, try to write out all possible Tic Tac Toe games. Then convince yourself that "O" can force a tie if he/she plays in the corner and "X" can force a win if "O" plays in a middle. Use rotational and mirror symmetry to make your search simpler.