Wanted to add, Fishlore was one of the places I went to when I first got back into the hobby and that whole site is terrible. Honestly I wish it would get shut down, some of the information I got from them resulted in several dead fish until a friend told me about MFK... I distinctly remember being told that a blood parrot, a common pleco, and a jack Dempsey were a perfect set for a 35 gallon hex tank... the pleco is the only one that survived and got to a new tank.That tank is perfect for an Oscar. They don't grow to 18". Now a day if they did grow that size, there would be pics. It's a bit irritating this myth is still going around. It's seems to be perpetuated by people on forums who read or heard that somewhere, and keep repeating it. Most of these folks have never owned one themselves but hey will argue it till they are blue in the face. Lesser forums like "fishlore" seem to be the leader in this false information.
Regardless, its sounds like you have this figured out. Good food and clean water is all they need. Provide that, and you should have a nice 12" Oscar before you know it.