I posted in a thread with some monstrous oscars...point is, once they get in the 15 and up range what difference does every inch make? I worked downtown where there was a public aquarium and they had hulk oscars, the SMALLEST ones were 14 inches. Most of the were at least 7 years old. I didn't know anything about Os at the time so to me they were just big fish.
Some things to look for witth giant oscars. Head/body ratio, super high backs, distance between eyes (thickness). Those things you can't fake. And they glide in the water like leviathans, s-l-o-w. They also get kinda dull with age color wise. Jaws get super thick.
Tell ya what...I wanna see how big I can get Brick. If you have an O I would say get them as big as possible that first year, since that's their highest growth curve. Then time and genetics will take care of the rest with a big tank and proper care.
King Zeus the tiger Oscar (Youtube) was 15 inches at 6 years, still growing (no longer alive). Owner had an earlier video of Zeus at 12 inches and you could definitely tell the difference.
Bug the giant Oscar (Youtube) 16 inches at last count
I think the videos are still up. Zeus made a silver arowana jump the tank. Both were raised in big tanks with minimal tankmates or solo. Those are two I can think of off the top.