Oscars should live for 10+ years in aquariums but it comes down to the owner keeping the tank clean. I know of a number of people that have Oscars that are over 10 years old and the fish are doing great.
Clean water, clean gravel, a clean fliter, and a varied diet will go a long way to keeping any fish alive and well.
The biggest killer of Oscars is hole in the head disease that is caused by a parasite called Hexamita. This disease thrives in dirty tanks. Because Oscars eat a lot of meat, they pollute the water quickly and this encourages disease organisms to thrive. If you do big (75-90%) water changes each week, and gravel clean the substrate when you do the water changes, and clean the filter regularly (at least once a month), the tank will remain clean and the fish should never develop hole in the head disease.
The fish should be fed a variety of foods ranging from dry pellet foods, frozen (but defrosted) foods like prawn, fish & squid, and live foods like earthworms, insects and insect larvae (depending on the size of the fish). Feeding the fish a variety of foods stops them going on hunger strikes if their normal food isn't available, and it provides more nutrition so they are less likely to suffer from health issues associated with malnutrition.
The minimum size tank for an adult Oscar cichlid is 4 foot long x 2 foot wide x 18 inches high.
