Tetris Best 2 also allows O-spins, but only with a diagonal slide, as in this diagram:
Games that count O-spins towards the score count it the same as a T-spin with the same number of lines.
SRS has O tetrominos move into a suitable position if the player tries to rotate them when one of their tiles is being overlapped with another mino. This can be used in Puyo Puyo Tetris' fusion mode to have a line be cleared, dropping the tiles above onto an O-tetromino that is currently being dropped. Then the O-tetromino can be rotated to fit into a 2x2 spot. A link can be found here.