Looks like HITH to me too.
And while meds are a pretty good temporary bandaid, unless you get to the root of the problem, HITH will just continue to reinfect, over and over.
To get to the root, water changes, lots of them, and (as suggested above) at least 50 % twice per week forever.
What water changes do is dilute chronic disease causing nitrate, which it seems oscars are susceptible to.
And if your water is hard with high pH, the bacteria that cause HITH are more virulent and ubiquitous.
In the rivers oscars naturally inhabit, nitrites are usually undetectable, and because the water is also soft with low pH, those HITH bacteria are not as prevalent, so no need for oscars to build resistance to them.
To me any nitrate level above 5 ppm is too high, so I maintain undetectable nitrate levels in my tanks
unless of course your water is naturally soft with pH below 7.
Below is how my average nitrate test comes out (the tube on the right)
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Below a test of the water where I catch my cichlids
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