1. The double threat (a โforkโ)
The oldest trick in tic-tac-toe still wins here: set up two threats at once. Your opponent can only block one โ so the other one wins.
X threatens the top row and the left column โ the two green cells win. O can block one; X plays the other and captures the board.
๐ฃ On the big board it's even deadlier. Capture boards so you threaten to complete your three-in-a-row in two places at once โ no single move stops both.