You need to know the water parameters, someone telling you it is fine is NOT doing you any good, what are the EXACT numbers? I have been to many petsmarts (in various states) and everyone I have been to uses test strips that do not register ammonia, The petsmart guy may be right but how do you know?? The very real possibility is that your tank is in the middle of a cycle, if this is the case without diligent water changes your oscar will probably die. You need to buy your own test kit and if your measuring ANY ammonia or nitrites you will have to do daily water changes. I will agree that with the current size of your fish you are fine on tank size, you have said many times you plan on upgrading, I kept 2 young oscars in a 30G for a few months with no issues until I was able to set up my 90G, but diligence with water changes is essential even when they are small.