James said:
what would you suggest to mix with an oscar?
How big is your tank ? How much do you enjoy performing water changes (seriously) ?
If you've got a big tank, there are many many fish that do well with Oscars.
If you've got a somewhat smaller tank, but your idea of fun is changing water every few days - well the list of tankmates is smaller but still possible.
If this is the 10g tank listed in your tank list - the only thing I would "mix" with your oscar is a bigger tank.
Preferably at least a 75g tank.
I've got a large, 10+ year old, 12+ inch long, oscar in a 125g tank. Tankmates include loaches, catfishes, a gold gourami, siamese algae eaters, a flying fox, and green severums.
The Oscar doesn't bother anybody who isn't between him and his food(pellets, krill, worms, never ever feeder fish).
Water changes are 50%, weekly.
If I skip a week, my oscar starts showing signs of hole-in-the-head -- open sores on his forehead. Not pretty and not healthy.
In a smaller tank, or if I were less comfortable doing water changes, I'd have less fish in the tank.
I've kept smaller oscars in smaller tanks - but a "smaller oscar" doesn't last for long, mine have usually grown about 6-8 inches in 9 months. At that size its hard to keep the healthy small tanks...