An Oscar fish will quite easily reach 12 inches in as little as 18 months. They are also bulky fish that can reach weights of over 2LB. However if you want to give your fish the best chance in life, give it a healthy environment to live in then it is absolutely paramount that it starts its life off in the correct sized aquarium. Keeping any large fish in a tank that is too small will almost certainly result in poor water quality. Oscars produce a large amount of waste that can easily take hold of an aquarium if it's not big enough. Any fish that is forced to live in dirty and polluted water will either die, or will become unhealthy. Even if your fish doesn't die straightaway, exposing fish to poor water quality can damage the immune system which may mean that your fish develops problems later on in life. Oscars that have been kept in aquariums that are too small will often not grow properly and may become stunted. So instead of your adult Oscar reaching 11 or 12 inches, it may not get any bigger than 6 inches.
People often ask how big the aquarium should be for one or two Oscars. My advice would be to give your fish the biggest aquarium that you can both house and afford. The rule of thumb and what is uniformly accepted throughout the Oscar fish community is that a single adult Oscar must have an absolute minimum of 55 gallons to itself. This means the fish must have the entire aquarium to itself, no tank mates at all. So if you want to keep even one tank mates as well, you're going to need to start looking for aquariums in excess of 75 gallons. Once you start getting into the realms of more than one Oscar, you're looking at aquariums sizes of 100 gallons and upwards. It shouldn't be difficult to do the calculations, the more Oscars you have the bigger the aquarium. Four Oscars will require an extremely large aquarium of at least 200 gallons. Also remember that large aquariums need to have filtration to match. Once you start getting into the world of really big fish tanks, you need lots of filtration which doesn't come cheap. You will also need to heat your aquarium as well, a 55 gallon tank can be heated easily with 200 W. However once you start getting over 150 gallons, you're looking at a fair amount of heat that will need to be running 24/7. Remember that all this is going to cost money, lots of money if you are heating large amounts of water.
Before you think about buying an Oscar, make sure you ask yourself a few questions first. Are you going to be able to provide a tank that is big enough? Can you afford the electricity that it will take to run the aquarium day and night, every single day of the year? Oscars are adorable fish and it's no wonder people fall in love with them. Unfortunately it's rather like puppies, people often get Oscars without putting much thought into how they are going to look after them. Remember that an Oscar fish can live as long as a dog. So if you are getting an Oscar you are committing yourself to many years of weekly maintenance to keep the fish healthy. Keeping the fish in a large aquarium will make your job that much easier.