Solution: AC and BD are equal. Connect OC and OD. By proving that triangle OAC and triangle OBD are equal, it can be proved that AC and BD are equal.

Given OA = ob, then the triangle OAB is an isosceles triangle, so the angle OAB is equal to the angle OBA. OC and OD are the radii of a circle, so they are equal. Therefore, the two triangles OAC and OBD are congruent, that is, AC and BD are equal.