two oscars will grow about an inch a month each until they reach about 8" long, then they will slow down a bit. by the time they are adults (8-10 inches, but they can reach a max size of 12-16") they will need at the absolute bare minimum a 100g tank. 150g would be much better though. this is for several reasons.
first, oscars are very territorial. they will fight and need plenty of space to have their own territories. its best to get a tank that is as long as possible, at least 18" deep front to back, and somewhere around 18-24" tall. if the babies are raised together, they have a much better chance of getting along as adults.
second, oscars are VERY dirty fish and produce a lot of waste. they need a lot of extra water volume to help dilute the waste they produce, otherwise you will be doing water changes every day to keep the nitrates down. this leads into the third reason, which is that oscars are VERY sensitive to their water quality. the nitrate must stay below 20ppm or they will become suceptible to Hole in the Head disease (HITH) which is basically when the fish develops large pits and deep scars in its head, which continue to grow. sometimes it can be stopped, but i don't think it can ever be reversed. this disease is only found in large cichlids that have a poor diet and poor water quality.
unless you plan on getting a 110-150g tank within the next 2-3 months, i'd suggest taking the oscars back and getting something more suitable to a 20g tank.