A big plec is about it, but you'll want to buy it big alread since they grow much slower then Oscars, and if an Oscar tries to eat a plec it will cause problems for everyone involved. A plec may take care of some algae but will contribute considerably to the waste, so it also depends on what you mean by 'cleaner fish'.
Ive got Striped Raphael Cats with my oscar.... they seem 2 get on ok after a couple of teething problems @ the start... ie, oscar trying to drag the cats around by their fins. My oscar found them a lil too spikey for his liking in the end. Now they all get on great. Whilst my Oscar is in his usual night time trance the cats get to work on their cleaning! Works for me!!!!!!
Only thing is... re-arrange the tank if you put cats in then the oscar doesn't feel the cats are intruding on his territory!
Happy cat hunting!