This Ok icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,700 icons, all in the same style and quality.
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.