I'll point out some general concerns of mine.
A 55gal is not a forever home for an Oscar. If you already have plans great. If not make them soon. Oscar's grow fast and in my opinion should be moved up to a 6ft 125gal when they hit the six to eight inch mark. Also I think your temps may a bit high. I keep mine at 75 and have no issues at all. If you think about the rivers they come from are not 82 year round. Yes in the summer months, but does cool off in winter.
as an FYI clove oil is to put a fish down, not to treat it. Not knowing what it is for sure polyguard be the way i would go, as stated tank small for an Oscar, water changes would need to be to be least 90% or fin level twice a week. link below for polyguard.
keep in mind it is broad spectrum and may send your tank into a mini cycle.
Diet, mostly the floating sticks, which is a live food alternative and an occasional night crawler. He hadn't gotten the night crawlers in a while because of the nitrate levels. He didn't take to kindly when I would put greens in there for him.
Jexnell, I do have plans for a bigger tank for him and I will lower the temp back down as I had raised it a few days ago thinking the 75-76 was too cold for him.
pops, in my research, I read that clove oil, in small amounts, could be used to sedate, and larger amounts to euthanize. Also, in my research, learned about quality foods (and the preservative called Ethoxyquin) and about a week ago, I had tried to start him on Hikari pellets, (which I feed my Blood Parrots) and that's what I made his medicated food with, Hikari pellets, garlic guard and focus, he isn't eating anything right now, not even the sticks.
Don't recall seeing polyguard, will look into it, thanks!
The dose for Epsom salt should be a 1 tbsp per 5g. Can use the polgaurd and see how it responds. Would hold on feeding unless it takes some food ( try metroplex) soaked in Epsom salt sol (1/2cup distilled water and 3/4tsp Epsom salt) can use seachem focus to bind everything into a paste. Numerous causes unfortunately esp since u mentioned it’s one sided. Sounds almost like a tumor but other things need to be considered. Can’t tell from the pics. Need to keep the water super clean.
Clove oil can be used for sedation and/or euthanasia.
so curious as to why sedate? clove can be used as a sedation, I knew this but only to sedate before euthanizing with say Alka-Seltzer I am wondering why sedate for treatment? This is something new to me as in treating an issue with sedation.