Ultimate Tic Tac Toe XO is a free board game for Android smartphones and tablets. It is one of the best and most popular games for Android devices. The gameplay is very simple and easy to learn. You just need to place three marks in a horizontal, vertical, or diagonal row to win the game. If you make three marks in a row, you will win. You can play the game with your friends or with the AI. You can play against the AI if you want. You can choose the grid size. The default grid size is 3x3. You can also change the AI difficulty. The AI difficulty level determines how difficult the AI is. It can be Easy, Medium, Hard, or Very Hard.