I noticed that my opponent has three X's forming a diagonal from (2,5) to (4,3): specifically, X is at (2,5), (3,4), and (4,3). This sets up a winning threat if they drop an X into column 5 (which will fall to row 2), completing a four in a row diagonally. Thus, I must block this threat by dropping my O in column 5, ensuring that square is occupied and denying the win.