First off a 55gal tank is really the absolute, barebones, rockbottom minimum for a adult Oscar

...
By himself, no tankmates

...
But, its still like you living in a small closet :mad2
A Oscar is a very very messy eater/pooper!

!!
It sounds to me like your water chemistry got wayyyyy out of whack and the toxins poisoned the fish

, causing the various symptoms you described.
I'd be willing to bet your poor Oscar and tankmates have been living in swirling toxic maelstrom.... FISHY HELL :thud:
Has your Oscar lived in this tank his entire life? I would love to see a pic of him. You may not realize it but he may well be suffering from stunting, deformities and/or disposition to disease resulting from a inadequate enviroment.
This could be the proverbial straw that broke the Oscars back, he may never be the same again.
OK, consider yourself well flamed

, now lets work on your fish.
A solo adult Oscar in a 55gal with really good filtration with atleast 10x turnover will IMHO need bi-weekly gravel vacumnings with 10-20% PWC's and the filters cleaned of solid waste buildup every other week.
This would probably succeed in keeping the NO2 levels at or below 40-50ppm.
I speak from experience

..........
You should be testing your water chemistry weekly for NH3, NO2, NO3 and PH.
You've been very lucky that "You" seemingly haven't had problems, but I bet your fish have...
Now your both miserable and its time for you to take action.
If at all possible, please get atleast a 75-100gal to even consider keeping the Oscar and Knife together.
Get your water chemistry under control, NH3-0. NO2-0, NO3-0-40ppm and a steady PH of 6.5-7.5 (Ideal) and institute a weekly/bi-weekly tank maintenance schedule similar to what I outlined above.
Treatment for current condition; I would get the water chemistry straight and then add a little salt, some Melafix and observe closely.
Start feeding your Oscar a varied nutritous diet.
Man... errr... Fish should not live on pellets alone! Pellets, carnivore sticks, krill, fresh or freezedried crickets, earthworms and mealworms, algae wafers, bottomfeeder sinking wafers, shrimp pellets, bits of fish and shrimp, bits of veggies and etc...
A good high qaulity cichlid pellet/stick should make up atleast 75% of his diet with other foods given as supplements/treats.
Notice I didn't mention feeder fish? Good, don't bother with'em
Sorry to hear about you losing your two kennyi (spg?) :sad