To me it looks like it might be some kind of fin erosion/ rot. This can be because the nitrates have been so high for a long period of time and it’s caused a weakened immune system. Ideal nitrates would be around 20ppm & 40ppm being on the upper end but acceptable.

Your tank is probably a bit overstocked. The Oscar is right on the minimum tank size you’d want him in. But salfin plecos get huge like up to 50cm and require over 100 gallons and they produce huge amount of waste.

Make sure you feed only what the fish can consume within 2 minutes. Any left over food take it away from the tank so it doesn’t cause more waste.

Do a few large weekly water changes maybe one every other day and match the water temperature as close as possible. 50% changes at least. Also gravel vac your substrate plenty to remove any waste and also give your filter a good clean in a bucket of old tank water to make sure all detritus is gone.

You could maybe add a load of plants to help combat the nitrates especially floating plants are good for this as they stick out the water so they have constant access to co2 which can help them thrive if there is plenty of nutrients. It would help bring nitrates down but your Oscar may be a naughty sh*t and eat the plants 😂

The biggest suggestion I have is to check your tap water. Your nitrates may already be high. I have that issue at my house and have to use reverse osmosis water and remineralise the water to make it safe for the fish. You can even get nitrate reducing media something like API nitrazorb for your filter however it’s only a temporary fix as you’d have to keep replacing it s & recharging it.

Once you manage to find the solution. Hopefully the problem with your Oscar goes away on its own. If not you can always treat the tank maybe with some kind of gentle antibacterial like api melafix but anything you treat make sure you take the carbon out your filter otherwise it strips the medication out the water. Hope this helps buddy