The best way to look at this is to get the biggest tank you can fit and afford and do not get an oscar unless it is a standard 75g or bigger. With the minimum tank size comes the need to over filtrate the tank and keep a very strict water change regime. Oscars are poop machines after all.
The reason that you do not see many 15" oscars is because they are the most abused freshwater tropical fish and many either do not see even 12", they are sold on because the person didn't realise they get so big or they are stunted because people keep them in 55g tanks or smaller for life and they assume that just because they survive then they must not get that big after all. Surviving is not living.
There are examples of very large oscars on this site, it is just a matter of finding them.
Remember that people consider keeping a fish alive for just a year or two as success not realising that oscars can live for over a decade with the right care.
2x Overfiltraion ia aways a good thing, the more the better. A strict water change and cleaning schedule should always be adhered to issure stability within the waters gravities. 75 -min. Filtration rates and added dissolved oxygen levels will increase success with tankmates. 125 long will give you more future options. The suggested 120 - 4x2x2 should be researched prior to purchase as some don't have tempered glass bottoms and can and do crack.
I am planning on getting a 75 - the 55 was put on there just out of curiosity. I plan on getting a canister filter with 2x filtration as well as a HOB with as much of a turnover as I can find. I am intending on a minimum of 2-3 water changes a week. Since a few of you seem to think I should pair my Oscar up, how do you sex them? I don't know that I will go this route, but I am just curious.
If youre "planning" on getting a 75, "plan" on something bigger if you have the room. You wont be able to keep anything with it in a 75, at all...and that is such a bare minimum.
In the NY/NJ area, there are usually TONS of tanks on craigslist way bigger than a 75 for under $200, even with stand, filter etc.
To sex an Oscar, you basically need to flip it over and vent it. There are no characteristics that favor one sex or the other.
Out of the choices a 125 would be great. A 75 would work but take more time and effort to maintane. I think a 90 is a good size same footprint as the 75 with a little more volume and height.
How do you figure? I had two Albino RTOs that were both 13"+. I rescued them, got them healthy, and then sold them to a guy who had a 200g waiting for them. I had them in a 55 for about 3 weeks and you could tell they weren't too thrilled. I suggest a 75 minimum for a single fish, but as the general consensus shows, bigger is always better.