RMO 2013 D1

Let $\omega$ be a circle with centre $O$ . Let $\gamma$ be another circle passing through $O$ and intersecting $\omega$ at points $A$ and $B$ . $A$ diameter $CD$ of $\omega$ intersects $\gamma$ at a point $P$ different from $O$ . Prove that $\angle APC= \angle BPD$