Doing lots of large water changes (how large is the tank, and what other tankmates currently?) because fresh clean water boosts fishe's immune systems, thus helping them fight of secondary infections like fungal infections, could be the best first aid.
Is there any chance of setting up a QT arrangement for him? Only needs to be something small, like a 2.5g, or even a new, unused plastic storage tote with heater (placed on stones/hardscape so as not to scorch the plastic), or a cheap second hand set up from Gumtree or something. Then something like a small, inexpensive (or second hand) sponge, internal or HOB filter, and lots of live plants, or just an air bubbler and a decent amount of fast growing live plants and some decor taken from the main tank to provide food, would work, be easier to treat and do daily water changes on, and give the fish a chance to recover from any bullying he's receiving in the main tank.
Since the first oto had a chunk taken out of his tail, and the synos or betta could easily have hurt both otos, I'd recommend a QT set up, large daily water changes, and a course of salt water treatment.
If it must be in the main tank, then W/Cs and salt treatment is still possible (see "emergencies" section of forum to find salt water treatment plan and dosing). Salt has antifungal and antibacterial properies, so can make a big difference in only a few days. However, it does still put some stress on the fish, and more delicate plants too, so better to isolate him for treatment if possible. Other meds also stress fish and can soon get very expensive, and many are snake oil, so unless the problem is something contagious that requires the whole tank being medicated, then it's better to isolate the fish (fish will also bully and chase away a sickly fish, stressing it further, even usually peaceful fish will do this, since sick fish attract predators), and be able to W/C and medicate the smaller volume of water. Doesn't have to be a permanent QT tank set up, can be temporary, but very useful.
