Ok one, I don't keep SA cichlids, but my good friend is probably the largest breeder in the state for chichlids of any kind. My best friend and next door neighbor has several tanks of SA's and I know the basics of the "fish world"
1. Oscars are messy eaters, they need to eat a lot, and they are not so great at getting nutrition out of their food, causing them to have a higher intake and thus poop more. Getting really good food will help.
2. Plecos are armored fish. SA cichlids aren't biters as much as they are "thrash and smashers" I'd put dollars to doughnuts that a pleco of any kind would be just fine in the tank as long as it had caves to hide out in during day time. (drift wood and some stacked slate would work nicely)
3. Fish don't "grow to the size of their tank" their growth is stunted by poor conditions. Oscars are fast growers, and I wish petco/walmart/insert chain pet store here would stop selling cute baby oscars to people who can't properly care for them. Add common plecos to the list of fish i wish they would stop slinging as well.
4. a 55g is questionable for an oscar anything less is unacceptable. Just come up with a plan for what to do with your fish in the event he gets too big, new tank, pet store? etc.
5. your filter doesn't really remove waste... yeah its not like cleaning the poop out... bacteria does that. sort of...
in the end the OP already has an oscar and the could of would of should of's don't really matter as he has the fish! So lets be supportive and suggest solutions instead of picking at him.
ottos etc are not going to work well with an oscar (if it fits in his mouth he will eat it) they also need pretty good water quality, your oscar isn't really the ideal companion fish for anything finicky
Pleco, Snails, or magnetic algae cleaner (these don't eat at all