Is it normal for your Oscar to lay at the bottom of the tank if you just got him today and he is in a 55 gallon tank with great filtration and aeration?

Check the Ph of the water, which will require a Ph test kit from the tropical fish store. Also, did you dechlorinate the water? Did you allow the water in the bag to adjust to the water temperature in the tank before you let the Oscar out? Do you have a water heater, and is the water temperature at least 78 degrees?

It sounds like your fish is in shock. There are a number of things which can cause this, including those I have listed. If all of the above have been dealt with, ask the dealer what might be wrong.