does anyone know how large of a tank can be used for ONE oscar with no other tankmates?

40-50 gallons would be a minumum for 1, and we're talking absolute minumum. that jumps to 75-100 for a pair. also, indestructable decor, a really good filter or two, and very regular maintenance would also be required, as these are messy fish.

were you looking at a specific type of oscar?

I absolutely love oscars and I think along with goldfish, they are some of the most commonly stunted fish I have seen.

I guess a 55 would be the bare minimum, but really they will not be that comfortable in such a small space. Like Bala mentioned, 75-100 is more like it, even for just one, I would say. These fish can get well over a foot long, but they have enormous mass as well, like a goldfish.

They are awesome fish and I have even heard of them being kept in outdoor (heated of course) ponds. I think a pond would really be the absolute best way to keep them as that would be plenty of room and the filtration could be more powerful as well (both bio and mechanical filtration).

Definitely consider all the factors before buying a cute 2" baby oscar, and keep in mind that they can double that size in about a month and even a 20 gallon is not big enough for them at that size.

Also note that they SHOULD NOT be fed a diet of feeders, as goldfish are very fatty and unhealthy and promote aggression. Oscars are a rather docile cichlid and need a varied diet just like goldfish, with pellets and fresh veggies.

For a single adult oscar I would recommend nothing smaller then a 50 or 55g. Oscars can grow between 12 and 14". A 50g will provide more turning room and less length while a 55g will be longer but less wide. Anything less and you will probably run into bio-load issues. Oscars are sensitive to high nitrates and are prone to HITH (Hole in the Head). For two oscars I recommend nothing smaller then 75g with larger being better.