Looking at the current board state:
O | O | X
---------
X | X | .  <- This is position 1,2
---------
O | O | X

I need to place my X at position 1,2 (middle row, right column) as it's the only valid move available. This move will complete a diagonal line of Xs from top-right to bottom-left (positions 0,2 → 1,1 → 2,0), giving me three Xs in a row and winning the game.