Keeping oscars in a 55 gallon is a bit like saying can I keep my kid in the laundry room. Sure, it can be done, it just dosent mean that its the best thing to do. A 12 inch fish dosent have much room to turn around in a 12 inch tank, plus he can only swim about 3 lenghts before having to turn around. Now if you know you are going to get a bigger tank soon, and those things grow fast, then get a small oscar, and buy a 90 gallon in 6 months or so. I think that buying a fish and planning to get a bigger tank when he outgrows it is stupid, but if you have the money, and know you want to it can be ok.