O mark is the name of the circle symbol "◯". It is often used in East Asia to express affirmation. Its use is similar to that of the checkmark ("✓") in the Western world. Its opposite is the X mark ("✗" or "×").
The symbol has a variety of names and meanings in various cultures. In Japanese it is called marujirushi (丸印) or maru (丸) and expresses affirmation. In Korean it is called gongpyo (공표; 空標; lit. ball mark) and expresses affirmation.