Which type of arowana are you asking about?
If you are planning on a silver arowana, personally I would keep it by itself until its 6-7 inches. They are very skittish while young and can be easily stressed. It's also probably best if you get a silver that's around twice the size of the oscar.
My current silver I got at 6 inches and my Tiger Oscar was also around 6 inches at the time and even though they were the same length, the sheer mass of the oscar made it a much larger fish still. The oscar did pick on the silver and made it jump around so I had to put a divider in between for a bit. I removed the divider when my silver hit 10-11 inches and my oscar was about 7-8 inches and by that time, my silver had gained some mass as well and was used to his tankmate. They now get along fine and like to steal each others food even. Hope that Helps.