import random

def ai_move():
    for i in range(3):
        for j in range(3):
            if board[i][j] == " ":
                board[i][j] = "O"
                if check_winner():
                    return
                board[i][j] = " "
    while True:
        x, y = random.randint(0, 2), random.randint(0, 2)
        if board[x][y] == " ":
            board[x][y] = "O"
            break