In a 55 you will find not much will work as the oscar grows and even as the only fish in there when he's full grown it will be a tight squeeze, as for now you could try a Convict or Firemouth, Jd if you plane on upgrading the tank to a bigger one as they grow, if not I'd get some smaller schooling fish that as the Oscar grows he can eat to clear room.
you can go with bottom dwellers like Cory or another one of the smaller cats but stay clear of the common plecos, they dirty the water fast and grow way too big for a standard 55 gal.
Hope that helps.