Oscars are a cichlid, and cichlids (along with their damsel cousins) are perhaps some of the smartest, most personality driven fish around. Cichlids are the most successful parents of any species on earth it turns out... considering the number of offspring that they often raise with such low mortaility rates (I heard that on PBS once). Of them, the oscars are very personable. They are very smart. In the wild, they can be observed teaming up, using tactics to hunt for food. The oscars will circle a crayfish/freshwater lobster at opposing sides... so the pincers are spread out at each of them. Then, they will both lunge at the shellfish at the same time, each grabbing a claw in their mouth and ripping the critters claws right off!! Then its easy snacking.
I used to be an african cichlid breeder myself, and its no secret that part of the attraction for this very large group of fish keepers is the personalities of the fish.
Heck, you want a real firecracker of a cichlid for a 10-20g tank? Get some tanganyikan cichlids called shell dwellers. You can keep a flock/harem of them in a 10-20g, and they are very smart little buggers that are fun to watch. Not as much interactive though as just interesting.
For saltwater, the only personable fish I know of are large, with few exceptions. One exception would be Jawfish. They are like little pitt-bulls of a nano-reef if you get one. I love em myself. They have attitude. And they will interact with the owner somewhat.
Other options would be a peacock mantis shrimp. These things are fierce, and you will never pet it, but when you look at them, they look right back at you (trying to decide if it can kill you all together, or if just a finger-snack will do). They are unique in many ways, and if you want proof...
http://www.reefcentral.com/forums/showt ... did=266915
Or an octopus... but those can be challenging... too challenging. But smart as all heck. They have little to no known predators, can catch sharks much larger than they are (and can eat mantis shrimp), and have even been recorded leaving their tank, going across a room to open a jar of food, then closing the lid, and going back across the room back into their tank... smarter than your kids might be (they dont put the lids back on, right?)