Draw lines AD and DC such that AD||BC, AB||DC AD = BC, AB = DC

ABCD is a rectangle as opposite sides are equal and parallel to each other and all the interior angles are of 90º.

In a rectangle, diagonals are of equal length and also these bisect each other. Hence, AO = OC = BO = OD

Thus, O is equidistant from A, B, and C.