Oscars are very sensitive to water quality, and research in aquaculture has shown that 20 ppm nitrate is a stress threshold where diseases can become problematic.
If it were me, i would at least double my water change schedule and quantity, and filter media cleaning, to bring down nitrates to 10 ppm or less.
In nature Oscars are omnivores, so just in case your oscars diet is very high in protein, I would try to add more vegetation oriented foods.
Back in the 1960s when I first started keeping oscars, terrestrial animal foods such as chicken were erroneously recommended for fish, and my oscars always had HITH.
Once I realized these foods were very hard, and non digestible for fish, and the non-digestible fats quickly degraded water quality, HITH became a non issue for me.