In a rectangle, all the angles are right angles or 90∘
OB bisects ∠B and OC bisects ∠C
Thus, ∠OBC=∠OBA
∠B=∠OBC+∠OBA=90
Thus, ∠OBC=∠OBA=45
Similarly, ∠OCB=∠OCD=45
Since, ∠OCB=∠OBC=45
△ OBC is an isosceles triangle.
In △OBC,
∠OBC+∠OCB+∠BOC=180 (Sum of angles)
45+45+∠BOC=180
∠BOC=90
Thus, OBC is a right angled isosceles triangle.