Melafix isn't great. With fish that have damage you're just trying to prevent infection and further harm. Get ahold of some methylene blue and dose accordingly - it'll prevent infections while it begins to heal up. Normally aquarium salt helps prevent infections and yoking the heat up can kill bacteria and kick start the healing process (I'm not too sure about oto's sensitivity to salt, so be careful). The best thing you can do right now is cover your crevice so it doesn't happen again. I strongly suggest moving the oto to a small holding tank while you dose the methyl blue, salt, or heat so it doesn't mess with the other fish, plants, etc. Offer the little guy an algae wafer after a day and then after that give him some veggies (preferably cucumber).