If you do plan on keeping the fish, get a basic model gravel cleaner like the one in the following link and do a 75% water change and gravel clean the substrate at least once a week, and twice a week wouldn't hurt.
https://www.about-goldfish.com/aquarium-cleaning.html
If you can't find a basic model gravel cleaner then you can make one out of a 1, 1.5 or 2 litre plastic drink bottle and a length of garden hose or clear plastic tubing available from any hardware store.
Cut the bottom off the bottle and throw it away. Chuck the cap away too. Stick one end of the garden hose into the top of the bottle and run the hose outside onto the lawn. Put the bottle in the tank and get the air out of it, then start draining water out of the tank. Push the bottle into the substrate and lift it up a couple of inches. The gravel will circulate in the bottle and drop down, while the gunk gets drawn up and out with the water.
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Oscars are predators and do best when fed on meat foods. However, they need variety and should never be fed the same food every day otherwise they start to reject other types of food. You can give them pellet foods or frozen (but defrosted) foods, and if you feel a little barbaric, you can offer them live foods. If you use live foods like feeder fish, there is a chance of introducing diseases into the tank that can affect the fish so frozen but defrosted and dry foods are the best options.
Juvenile oscars like yours, will eat any sort of fish food and will take small pellets, flake food, frozen brineshrimp, bloodworms, daphnia, marine mix and any sort of small flies, moths or aphids, shrimp or fish that fit in their mouth. Make sure any insects are free of chemicals/ pesticides.
As the fish gets bigger you can feed the same sort of food but use bigger bits of food. When they are half grown most people feed them on fish bait like prawn/ shrimp, white bait, blue sardines, and various cichlid pellets. The fish bait can be bought from any fishing store but make sure it has not been sitting in their freezer for 6 months. If the bait is covered in frost and has ice crystals in the pack it is probably not the best. If you can't find good quality bait then buy frozen fish and prawn designed for human consumption and use that instead. Any sort of marine meat is fine including fish, prawn, squid, octopus, mussel/ clam meat, etc. Keep it frozen and take a bit out, defrost it and feed the fish as much as it can eat in a minute. remove uneaten food after that.
Baby fish should be fed 3-5 times per day. Adult fish should only be fed once every second day. You can feed adults every day if you like but don't give them too much food or they become fat and have health issues.
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If you want to have a couple of oscars, buy them now and grow up a couple of young fish. If you have an adult and try to introduce another adult there is a good chance they will fight.
Keep the water & gravel clean, and make sure there is lots of filtration and aeration on the tank. Lots of big water changes, a varied diet, keep the temperature around 25-26C and they can live for 10 or more years.
