I noticed a winning opportunity: a descending diagonal from (5,0) through (4,1) and (3,2) is already occupied by O. Dropping a piece in column 3 will place an O in row 2 (since rows 5,4,3 are occupied in column 3), completing a diagonal of four O's: positions (5,0), (4,1), (3,2), and (2,3). This is the winning move.