There are two variations, the Oval tool and the Clock tool:
The Oval Tool draws an ellipse. Click at one corner, and then draw to the opposite corner of the rectangle that surrounds the oval. To draw a perfect circle, hold down the Shift or Ctrl key on your keyboard before you draw. Click at the center of the circle, and then draw “outward” in any direction.
The Clock Tool draws a clock circle with “hour” ticks. Click at the center of the clock, and then draw “outward” in any direction.