There are different opinions as to what "good" water quality is.
Oscars originate in waters that have almost 0.00ppm nitrate, (so to me, any nitrate concentration over 5ppm in their tank, is not good water quality)(not saying that's what your have, just generic info) they also come from waters that seasonally have an influx of tannins, (which are anti-bacterial in nature), and help prevent these chronic type ailments, and if they don't get these conditions, don't do well over time, and get get stress born ailments like HLLE.
In many case their natural waters are also soft and mineral free, pH below 7, so if your water is hard, and mineral rich, and your nitrates are over 5ppm, this may be the cause.
Your 3rd pic looks to me like the beginnings of HLLE, in the area between the eyes.
Medication doe not solve the problem.
Prevention is the key, lots of water changes, lots of cleaning gunk out of filters, and the addition of tannins may be the best solution.