Oscars growth rate is around one inch per month till they hit around 9" then it will slow down, they reach maturity around 18 months. My pair are little over three years now at a length of 14"+ and weighing around 3.5lbs.
Feed them a quality pellet like New Life Spectum and their color and growth will amaze you. Water changes are also a important thing to remember regarding growth, maintaining pristine water conditions with large SA/CA
fish is vital to prevent certain diseases like HLLE
Most of the best looking and larger oscars I've seen are in larger tanks of 150g and up, not saying a single Oscar can't live in a 75g but in my experience the larger the tank the healthier the fish. Especially with oscars being huge babies and sulk about anything, so they really feel more comfortable and seem to sulk less in larger spaces. Buy who would be happy living in a closet for the rest of lives?