Hold piece

From wiki

Hold piece is a feature of most newer Tetris games starting with The New Tetris. The top left of the tetrion has a window called the hold space that displays a tetromino. At any time starting when a tetromino enters the playfield until it locks, the player can press the Hold button on the controller to move the active tetromino into the hold space and move the tetromino that was in the hold space to the top of the playfield. A tetromino moved into the hold space is unavailable for switching out until the tetromino that was moved out of the hold space locks.

What should I do with this O tetromino?

Press the Hold button