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's names and meanings vary across 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. zero mark) and expresses affirmation.
An X mark (also known as an ex mark or a cross mark or simply an X or ex or a cross) is either used to indicate the concept of negation (for example "no, this has not been verified", "no, that is not the correct answer" or "no, I do not agree"), or indicate affirmation (for example, in election ballot papers, legal documents, or maps as an x-marks-the-spot). When used to mean "no," its opposite is often considered to be the O mark used in Japan and Korea or the check mark (✓) used in the West. When used to mean "yes," it is often completely interchangeable with a check mark. In Japanese, the X mark (❌) is called "batsu" (ばつ) and can be expressed by someone by crossing their arms.
It is also used as a replacement for a signature for a person who is blind or illiterate and thus cannot write their name. Typically, the writing of an X used for this purpose must be witnessed to be valid.