Ok and other 68,900 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
There is a check mark, which is placed in the middle of a larger circle. The check mark looks similar to a reversed L, and is slanted so that the point where the two lines of the L meet is the lowest point. It is a standard check mark.