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O is a chubby, slime humanoid who has a slime "O" surrounding their face. Tiny specks can be seen inside of O's body.
O is the Slime Elemental, therefore they have the power to control slime.
- According to Steve Wolfhard, O was included in order to give closure on Slime Princess.
- O's name, along with X's, may be a reference to the game Tic-tac-toe (also known as Noughts and Crosses), with and O representing the noughts, and X the crosses.