in d vedio why do we don't take AO - OD and OB - OC as d radii ???

Dear Student!

The given video explains the theorem" Equal chords of a circle subtend equal angles at the centre of the circle".

It is taken in the video that, 

OA = OC (Radius of the circle)

OB = OD (Radius of the circle)

You can take, OA = OD, OB =  OC and AB = CD to prove that ΔAOB ΔCOD.

Cheers!