Now, I admit that this comment is based partially upon the fact that, as a Canuckistani, I have grown accustomed to a virtually zero availability of fish medications...but even back before the gummint realized what a danger I was and curtailed my ability to buy such things, I rarely if ever used medication. The invariable and immediate knee-jerk reaction to dose with this or that or the other thing makes me a bit nervous. "We don't really know what's wrong, so...try this...and then this...and maybe a bit of this..."
You have a 6-year-old Oscar who looks like a 16-year-old Oscar but is the size of a yearling Oscar. Such a stunted fish got that way through chronic exposure to at least one ongoing problem; lack of food (not likely because owners just love to watch their fishies eat), poor water quality (nitrate or likely nitrate plus other unknowns), poor genetics (but I think this fish's miniscule size was not caused strictly by genetics), or a combination of the above and possibly other factors.
I won't suggest what medications, if any, would help; literally haven't a clue. But I would bet serious folding money that a 50% water change daily for a week...which would get you down to near-zero, near-brand-new water...followed by an ongoing routine of at least 100% weekly would work wonders. The change won't occur overnight, although improvement should be noticeable pretty quickly, but you won't be maintaining a healthy fish...rather, you will be trying to provide an already weakened fish with better conditions that allow it to heal naturally.
Certainly, use medications if there is a reasonable diagnosis. But change that water! Whatever kind of filtration you have, remember that almost all filters are designed to cycle ammonia to nitrite to nitrate. Most stop there, and that nitrate accumulates; that's not good. And that's about the only thing you can test for and measure; there are countless other compounds and substances that fish is pumping into the water as it eats and respires and poops. We are led to believe that we can ignore them, but that literally makes no sense.
When you change 25% of you water, it makes you feel good to know you are doing good for your fish. But that means that you are leaving 75% of pollutants and toxins in the tank. The next week, you change another 25%...leaving 75% behind. Would you install a toilet in your house that, when flushed, only drained away a quarter if its contents? The accumulation of "stuff" is insidious, ongoing...and can only be addressed in one way.
Change. More. Water.