In the parallelogram ABCD, ΔAOB is congruent to
ΔCOD
AB = CD (∵ Opposite sides in a parallelogram are equal)
∠ABD = ∠BDC (Alternate angles)
and OB = OD (∵ Diagonals of a parallelogram bisect each other)
∴ From SAS congruency we can say that ΔAOB is congruent to ΔCOD