How long will they (Oscars) grow to get bigger? In a one or two years?
I would say skip the 30 gallon tank, mine grew from a little over an inch to 6 inches in a matter of months in a large aquarium. A thirty gallon tank for a fish already 3.5 inches will be tight. Oscars are VERY dirty fish and will destroy the balance of a tank, the bigger tank is a must.
The larger tank will also prevent ammonia reaching toxic levels as fast. Put together the tank and keep 12 small goldfish feeder for a few weeks to help start the bio cycles and then begin preparing for an oscar, it takes months to properly establish a tank, and a newly prepared tank will not support the oscar well. However my black convict cichlids do well work hard to clean up the mess of the wasteful fish foods on the bottom. When my Oscars do spitted coming out from their mouths as I noticed that my black convict snipped real quick of the jumbo carnivore sticks with the small chip bites.
Not to say that it cannot be done, it will just be difficult to do, my nitrogen levels will be all over the place for a long time and will not be terribly forgiving. I will need to closely monitor the water balance and be ready for frequent water changes to keep the fish waste from becoming toxic.
Recently to measure them on my 2 Oscars. They are 5 inches now. I bought them last spring time. Doesn't take very long for them to grow so fast.
LOL!