Had a moment in my last session where the Striker provoked an Opportunity Attack from a Monster that was marked by the Defender. After the attack was resolved, our Defender wanted to respond with his mark - Swordmage's Aegis of Ensnarement.
Our GM said that since this was an OA the mark wouldn't trigger. He reasoned that since forced movement doesn't provoke an OA; an OA doesn't trigger a mark effect since it is a forced attack.
Our Defender disagreed about the OA being forced and thought it was more of an option.