I once gave my mid-sized oscars a live feed of juvenile convicts. I now
have a large pair of oscars happily living with second and third
generation convicts, many of which are snack-sized but are never even
looked at twice by the oscars.

The trick is to always overstock predatory fish (and remember, oscars
are not predatory fish ordinarily, and in the wild, are considered to
be herbivorous), and make sure to have plenty of hiding spaces for
smaller fish, and only add them to the tank immediately after giving
the existing fish a big feed.