
ABC is a right angled triangle. O is the mid point of hypotenuse AC, such that OA = OC
Draw CD||AB and join AD, such that AB = CD and AD = BC
Now quad ABCD is a rectangle, since each angle is a right angle and opposite sides are equal and parallel.
We know in a rectangle diagonals are of equal length and they bisect each other.
Therefore, AC = BD
And also, AO = OC =BO =OD
Hence, O is equidistant from A, B and C.