Not a smart Angle fish! Oto's have very rough shark like scales, and I believe some bony spikes in their dorsal and pectoral fins, that will make them difficult to remove backwards as they may act like barbs.
Assuming the Oto is dead you would need to try and remove it with tweezers, possibly by pushing it a little deeper so the fins flatten, holding them down with the tweezers, and then removing it slowly. Very hard to do on a live fish I imagine, but if the Oto is stuck the angle may die otherwise.