Please see that , in order to draw a circumcircle we have to first draw the perpendicular bisectors of the sides. The point at which these perpendicular bisectors meet is called circumcenter and the it is the center of circumcircle passes through three vertices D, E and F.
This is because, When we draw the circle from these vertices, we can see that the three sides forms that three chords of the circle. And circle has a property that , the perpendicular bisectors of chords always passes through the center. And here o is the point through which all the perpendicular bisectors of all three chords are passing through. Hence 0 is the center
