Fish are opportunistic feeders (as are all animals including people) and will eat whenever food is offered because they don't know when their next meal is going to be. If you put food in the tank, they will eat until they are full and then eat again when they are hungry.
Big predatory fish like Oscar cichlids are usually fine being feed once a day or even once every second day. If you feed them a lot, then make sure you do big regular water changes and gravel clean the substrate. And clean the filter at least once a month.
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If you feed live fish and shrimp, you can introduce diseases to the tank so either quarantine the live food for a few weeks or use frozen (but defrosted) foods. You can also grow your own food in spare tanks or ponds.
Big Oscars can be fed on dry foods as well as white bait, blue sardines, squid, prawn and most other marine based meats. You can buy these items from the freezer at any fishing shop, or at the seafood section in supermarkets. Keep the items frozen and take a few bits out, defrost them, and feed them to the fish.
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If you have an Oscar that kills things, you might want to keep him on his own. Do not use aggressive fish for breeding because they pass on their aggressive traits to any young. Oscars are not normally a very aggressive cichlid but they are a large ambush predator and will eat smaller fish. They will become territorial during breeding and can go after fish that enter their territory.
As a general rule you should only have 1 adult Oscar or 1pr (1 male & 1 female) adult Oscar cichlids per tank.