Oscars (like Jack Dempseys) have a reputation as aggressive fish. That being the case, when people placed them with much more aggressive fish (red devils, festae, etc), many were slaughtered. This changed the perception of the oscar to where some describe them as "wimpy". In my personal keeping of oscars, I have seen them be aggressive feeders, but they are quickly bullied by others. My true parrot bullies my oscar and the oscar is nearly double the parrot's size.
At the lfs I used to manage, someone traded in a very large oscar. We measured it out of water at 15 inches lip to end of tail. This oscar grew up alone in a 90g and was incredibly aggressive to the point where we had to isolate him from causing additional trauma to larger parachromis.