Agree tank size plays a major part.I think the trouble here is tank size.
In a larger volume of water chances are the Oscar won't bother the pictus.In the confines of a 55 I think an Oscar would be tempted.
Size and age of both fish when introduced will certainly have an impact on the outcome.
I risked keeping rivulatus with Brochis splendens in my 7x2x2, there is no way in the world I would of risked it in a 55.
If i pass a chocolate cake in a shop window, I can resist,put it under my nose,it's gone.