Growing up with my father, who ALWAYS had Oscars, always had a pleco with his oscars and never had any issues at all. You can get Oscars and Plecos pretty small, but they both grow very large. But there shouldnt be a problem. Most middle and top swimmers (like Oscars) dont tend to bother bottom dwellers (like Plecos).
I have heard horror stories about Plecos becoming mean and killing smaller fish, but there shouldnt be an issue with the pleco and oscar, especially if there is tank room as they grow, and even less issues if you get them both as juveniles.
Hope this helps!!!