I am not a Vet, I did Research for you, that is all. My suggestion that you take it to the Vetrenarians is very strong, how ever before you do this, put in seperate tank, RO water, Filtration, correct heat, (all clean, or new)--
It looks like Hole in the Head Disease/ Freshwater Head and Lateral Line Erosion (FHLLE). It mainly effects Discus, Ciclids and Oscars. (It is a very dangerous disease, and you cannot ignore it). Although you say your perameters are perfect, water quality is a major cause, I wonder if your test kit is old or something.
Poor water quality is usually cited as the main cause, but also poor feeding (proper vitamins-- because there is also a vitamin aspect to this). Poor feeding might be the cause. What is your fish eating?
It is caused by parasitic larvae, Hexamita, and a following bacterial invasion.
If left alone, the lesions will eventually appear and then death.
To treat need to move fish to a quarantine tank with perfect water, (this is for any sick fish). It doesn't need to be an aquarium, a clean sturdy plastic container would do (brace container middle if it is big). Do not use charcoal in your filter because it will also filter out any medication. Possibly RO water because you have no time to cycle the tank.
(When the Oscar is in the hospital tank, you may want to thouraghly clean your tank, just for safety sake. This microbe (Hexamita) might be all over it. Hexamita can live in the bio-filter, or in the water of an aquarium. This is why you need to remove your fish and move it to a clean hospital tank, because it is being attacked by a parasite.
Dosage according to package with antibiotic metronidazole to the treatment tank housing the infected fish. Treat if secondary bacterial infections are present, with additional antibiotics such as Maracyn, Kanacyn, or Furan may be needed.
Feed good fresh food. Oscars are primarily carnivores. You might feed things like cut up (grocery store) fish, shrimp. I would steer away from live food because it might have some bugs in its guts.
Reduce all stress in fish. Keep place warm (to specifications for fish), lights low or covered. Treat like a sick animal-- which it is. Oscars are great, they call them the puppies of the sea. I would take it to the vet, if you do have a vet who looks at unusual/exotic animals. Many people have taken their koi and goldfish to the vets.
Once again, I am not a Vet, I am just an interested fish owner. Other people may have opinions on this.