I don't know about that, there's a 15", maybe 16" Tiger Oscar at the LFS.
Have you measured it yourself, out of the water? I'm not doubting your word but looks can be very deceiving, especially inside a glass tank filled with water your size perception might be way off. It's not uncommon to see a thick, tall bodied fish that is around 12" (which is very large compared to what one usually sees) and overestimate its size.
Most Oscars tend to average around 12-13" and there has been a few reports of specimens growing past 16" but it's not the norm. I think a 75 gal tank is enough for a single O, and that's the generally accepted minimum requirement.

