Circle mark

A circle mark 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.
The symbol's names and meanings vary across cultures. In Japanese it is called marujirushi or maru and expresses affirmation. In Korean it is called and expresses affirmation.

Usage in Japan

Japan interprets the symbol as an affirmation.
Japan employs a number of related symbols in a system that expresses degrees of affirmation. A bullseye "◎" is often used for "excellent", the circle is a plain affirmation, the triangle "△" means "so-so" or "partially applicable", and the "×" expresses disagreement. This system is widely known in Japan, and thus often used without explanation. Ad-hoc adjustments are usually explained.
The hanamaru is a variant of the O mark. It is typically drawn as a spiral surrounded by rounded flower petals, suggesting a flower. It is frequently used in praising or complimenting children, and the motif often appears in children's characters and logos. The hanamaru is frequently written on tests if a student has achieved full marks or an otherwise outstanding result. It is sometimes used in place of an O mark in grading written response problems if a student's answer is especially good. Some teachers add rotations to the spiral for exceptional answers.
Two circles ◯◯ are often used as a placeholder - either because a variety of words, names or numbers could be used in that position, or because of censorship.

Unicode

provides various related symbols, including:

Symbol	Unicode code point	Name
○	U+25CB	WHITE CIRCLE
◎	U+25CE	BULLSEYE
●	U+25CF	BLACK CIRCLE
◯	U+25EF	LARGE CIRCLE
	U+2B55	HEAVY LARGE CIRCLE
	U+2B55	HEAVY LARGE CIRCLE
	U+1F646	FACE WITH OK GESTURE

has both text and emoji presentations, as shown in the table. It defaults to emoji presentation.
The emoji looks similar to hanamaru, although it represents a rubber stamp commonly used to grade students' written answers and is not usually recognized as hanamaru.