a male convict will likely grow to 4"-5". although they are an agressisive fish, they would not be able to take down a oscar unless the oscar was small. a breeding pair is a different story..
keep a male in there with no female and they will be just fine.
Convicts are usually a lot more aggressive not to mention faster than oscars.
Oscars with convicts is a pretty common mix. If anything the oscars usually end up getting picked on, not the cons.
if the oscar is aggressive just provide spaces for the con that the oscar cant get into such as a cave or driftwood. if the oscar is larger its going to be fine with 1 convict.