There are different classes of aggression among cichlids. An INDIVIDUAL Cichlid that is defaulted (based on type of cichlid) to a particular category of aggression can sometimes not match people's expectations of them for example an oscar that ends up being an axe murderer, a midas that is a wimp, a dovii that gets beat up by everything.. etc.
To OP, Your oscar may very well be a tough fish but I'd be hard pressed to find any 6" fish , no matter how tough, that can hold their own against anything that is 14". Someone will probably pop up with their own story about a 4" buttikoferi That decap'd a 20" dovii. Those are special circumstances.. and not the norm.
Individual fish+ environment=individual personality