I'm choosing column 5 for two key reasons: defensively, it blocks a diagonal threat where my opponent has X pieces at (5,3) and (4,4) that could lead to a win if they played at (3,5) and later (2,6). Offensively, this move creates a vertical line of three O pieces at positions (3,5), (4,5), and (5,5), setting up a potential win if I can later play at (2,5). This is the strongest move that both creates a winning opportunity for me while preventing a potential winning sequence for my opponent.