I am really frustrated after reading an adult Oscar's growth can be stunted if they are in a tank smaller than 75 gallons? I had many nights of no sleep knowing my (still small) Oscar was in a 20 gallon tank and was going to grow out of it or have its growth stunted. So I went out and bought a tank that is just over 55 gallons. I can be a bit of a perfectionist sometimes and like to have things right for all my animals and it is very very VERY annoying after spending a load of money on my 55 gallon tank hearing that this might not be enough. Will my Oscar grow fine in this tank and is a 75 gallon tank only for more comfortable living or have I really messed up here?

55 will be ok.

you will need to be dilegent with your water changes. and make sure you have enough filtration. i would recommend keeping the decor down to a minimum.

what ends up happening, is your oscar gets to full size and by then you have come across a bigger tank somehow. (craigslist, etc...) and then you make your O even happier.

good luck.

The reason youll find that a 75gal is preferred is because while being 4ft long it is also 18" front to back. A standard 55gal is only 12" front to back and well kept oscars will typically always get larger than 12". So the narrow 55 tends to get cramped. Youll be good for a while but like Cebo said down the road you may want to upgrade.

You will always be looking for new tanks. its an addiction

Yea, tank shopping can be a big addiction...
a 55 will be ok, but if you can find one that is 36x18 it's better for the O. I have mine in a 55 and I'm going to try and upgrade this week to a 65 that has the same foot print. Remember that if you want a big happy O, there is no such thing as over filtration.