nah I doubt the plec would bother the oscar. I would definitely try to get a bristlenose though. they max at about 6" and should eat algae their entire lives whereas commons will grow far too large, produce way too much waste and at some point they stop messing with algae.
I made the mistake of putting a couple of 5 inch bristlenose catfish in with my Oscars a couple of years ago and they didn't last very long. One of them disappeared completely, never found out what happened to it, the other one just got chewed up and was left for dead. Personally, I'm not sure if it's a good idea to introduce these type of fish, even at 6 inches, a very large Oscar will easily get in its mouth. Remember, they may be 6 inches, but they are not heavily built fish. Just because it can't physically swallow it, it will kill it in a heartbeat.
