Here's my advice - Net the fish and remove him from the tank. Sedate him with clove oil until his gill movements are slowed and you can handle him. Use a wet, soft towel or washcloth to hold him with just his mouth out of the water. Best if there are two of you for this, one to hold and one to look in his mouth. Carefully open his mouth and look inside. Likely, he's already swallowed the otto by this point, but if not you may need to get some nail scissors and cut the otto in half, then use long tweezers to pull out the pieces. Sterilize both the nail scissors and tweezers first with alcohol or by boiling. Return the fish to a tub of clean water and watch him to ensure his recovery. Then return him to the tank.
Best of luck!