I wouldn't put any catfish with an Oscar in a 55... the size of the tank is marginal for the Oscar alone, and that's with excellent (read: 8-10 times turnover per hour) filtration. Anything too small to overload the system would be small enough to be eaten by the Oscar.
Edit: Also, look into Mag-Floats. They clean Algae, which is a plus, and don't poop, which is a bigger plus. Again, don't buy a fish to fix a problem! Research causes of Algae *Nitrate, Phosphate, too much light* and fix them.
Edit: Also, look into Mag-Floats. They clean Algae, which is a plus, and don't poop, which is a bigger plus. Again, don't buy a fish to fix a problem! Research causes of Algae *Nitrate, Phosphate, too much light* and fix them.