I'll place my "O" at position 2,0 (bottom-left). This creates a diagonal line of my symbols from the bottom-left to the top-right. With "O" already at positions 0,0 (top-left) and 0,2 (top-right), placing at 2,0 gives me a diagonal win with three "O"s from bottom-left to top-right. This is an immediate winning move that the opponent cannot block.