tokyoing.net/intro/os-and-xs/
Circles and Xs have clearly defined symbolic meaning in Japan. The circle has such positive associations as correct, passing, winning, and OK. On the other hand, the Xs stands for such negative things as mistake, failure, defeat, and rejection.
These meanings are conveyed in all circumstances, whether written or as gestures. On a true/false test, true is O and false X. The circle is often formed with the thumb and forefinger or by holding the arms upraised with the fingers touching to make an O.
помните, в "Воронах. Начало" Генджи пошел Руку на свиданку звать, а потом вышел и круг над головой изобразил?
в смысле "Окей, она согласна" ))