Reviewing the board, I noticed that the opponent has three X's stacked in column 5 (at rows 5, 4, and 3). If the opponent is allowed to place their piece in column 5, they would get four in a row vertically and win. To block this imminent threat, I must drop my O in column 5 so that it occupies row 2 and prevents the vertical connect four. Therefore, my move is to drop in column 5.