Bigger fish they cannot swallow and don't be fooled. Other cichlid types taht share the same aggression level such as blue acaras, firemouths, blood parrots. When getting an oscar it's good to get htem small and let them grow up with their tank mates so they don't try to eat them later. Bichers of appropriate size can live well with oscars too. Plecos and other catfish will do fine so long as they aren't small enough to fit in the oscars mouth.
Oscars are hefty fish and require lots of water volume. A single adult oscar should be kept in 60 gallon or more. In a 60 gallon, you won't have room for anymore fish. A pair, nothing less than 100 gallons.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.