I have had an oscar and an arowana together for years and the oscar occasionally tries to intimidate the aro then the aro backs him down. They are compatable for the most part and they never seem to harm each other
I tried once and my oscars ate all the food put in the tank seconds after it was in there so my arrowana didnt really get alot of food and died. I tried sneaking it in the back of that tank and stuff but my oscars are smart when it comes to getting food.