Good morning, electropunk. As an Oscar owner, I'd recommend at least 75 gallons for one Oscar for life. Bigger is better, though. Some even go so far as to say 90 would be the minimum, but frequent water changes make a 75 possible, too. Since your O might reach 15", try to find a wide tank, often called 'breeder style'. He'll need heavy filtration, too, both biological and mechanical. They are such messy creatures. Try to avoid buying the inbred specimens from chain pet stores because inbred fish are more prone to illness. Wild caught fish are more sensitive to waste but more hardy overall. I'd strongly recommend finding either a wild specimen or buying from a reliable breeder with a good track record for fish health.
Diet is easy because they will eat almost everything. The best advice I've gotten is a diet of about 80% high quality pellet food, and 20% live and fresh foods like crickets and earthworms (watch out for contamination from pesticides if collecting in the outdoors). While some people are determined to cram their fish full of food to encourage growth, rest assured that with light feeding, you cannot stop an Oscar from growing. As less food means cleaner water, the lack of chemical stress may actually encourage growth better than a non-stop gluttony. Try to keep the pH stable and near 7.0, and the nitrates below ten. The optimum average temp for an O is 77ºF. If you want stricter guidelines, head over to oscarfish.com. The helpful folks there are never shy about correcting inappropriate O care. Good luck!
You'll probably want to either buy a small O and seed the tank with beneficial bacteria from another tank or do a fishless cycle with pure ammonia if buying a large O because they are great at quickly polluting their water. A stable cycle is a must for these guys. Hope that helps.