Well, I dunno. I have small tanks at home, so I have small fish (the largest I have right now are full grown banjo cats).
Tanks without tankbusting monsters CAN still be fun. You didn't mention the size of the fish schools, if the tank is planted or not (I've found these two elements are key to a more relaxed tank of small tetras and the like...of course, if they are TOO relaxed, they don't school as much, it seems...), where it's located, etc. Take also into consideration that an oscar is, in the world of fish native to his area, pretty much close to the top of the food chain: he eats a good deal of other fish, which tend to be more jumpy and darty, IE, tetras and the ilk.
Or maybe you just need some oddball tetras! Just out the blind cave tetra; those little bastards will pull the hairs off your arm to see if it's food!
Also, while not monstrous in size, I used to have some multi shelldwellers. Now I had a small colony, and my dominant male died. Okay, suck. He was always out and around and curious as to what I was doing. For the next four months, the colony did not grow by one member under the "control" of the new dominant male. He was a very flighty fish, taking off into any shell whenever food came into the tank, and the females followed suit. I ended up hating the fish for this "duck and cover" activity which never ceased, even after five months of the new male's control. So I took them all to an lfs, and with shells, got over 70 bucks in credit. I now have a nicely planted corydoras tank with some small barbs, and looking to add in a dozen or two amano shrimp. The amano shrimp I have in another tank will scurry over when I'm working in said tank and start picking dead skin off my arm, a very odd sensation!
, i jumped straight in. But I'm into S/A's now and it's getting better