It looks like your Oscar has some kind of infection that has spread from a small wound site. Plus, it appears that fungus has taken advantage of that skin irritation and is now attacking the infection site as well. So to begin... I WOULD NOT go rushing off to purchase some sort of interesting 'fix-it' medication to treat your sick Oscar. More people seem to lose fish to over-treatment with medications, than almost anything else these days.
Recommend a treatment regimen as follows,
Water changes! You need to up the frequency and percentage of water you are pushing through this Cichlid's aquarium. Doing so will dilute the Nitrates, Phosphates - and most importantly - the BACTERIA growing out in your water. Kind of amazing how therapeutic a big infusion of clean, fresh water can be for an ailing fish.
Add NON-IODIZED salt to the water. Freshwater fungus and all sorts of problems can be eliminated from an aquarium by the simple expedient of adding salt to the water. You can purchase specialty aquarium salts if you prefer ($$$), or just plain old cheap table salt you can buy at the grocery store. Your fish won't care either way. Normally, I would recommend adding 1 tablespoon of salt per three gallons of water. In this case, you should be a little more aggressive with that and go with adding 2 tablespoons of salt per three gallons of water. Once the salt is added to the aquarium, only add the amount of salt needed to make up for the water lost in doing those water changes. Salt does not evaporate.
Keep up the water changes and salt treatment until you see noticeable results and improvement in your Oscar. Drop the salt addition to 1 tablespoon per gallon until your fish is almost completely healed.
In moving forward, you might want to purchase a Nitrate test kit for the water of your aquarium. Keeping the Nitrates diluted out below a measured 30 - 40 PPM in the water of your tank, will help to prevent a whole host of problems (Many MUCH worse than what you are seeing now) from happening to your Oscar. Plus, smaller sized tanks less than four foot (120cm) 90 gallon in size, are NOT recommended for keeping this species. One reason is that they are a large growing, incredibly messy fish and will foul the water of a smaller volume tank much faster than in larger sized aquariums. Another problem, is that a smaller sized aquarium will impose stress on a naturally open water fish like an Oscar. And just like in us humans... living with excessive stress in our lives can ultimately weaken our immune systems, and make us less capable of resisting disease or other problems.
It looks like heater burn that got infected. If that is the case, you want to add a heater guard to your aquarium heater by either purchasing one or making one using PVC pipe and drilling lots of holes in it to allow water circulation.