If you can go to an independant pet store where people drop off plecos and other fish that they don't want anymore, I would suggest getting a pleco that outgrew someones tank. As long as he is bigger than your Oscar, I don't see a problem.
As for the two Oscar thing...I don't recommend it. I have had a lot of experience with Oscars and found that having one happy, healthy Oscar is much better than having two that "get along for the most part"