Well, my Oscar killed my Arowanna (bigger than him) in less than 3 hours after living with her for 2 months. I still can't figure out what went wrong so I'd say NO. they aren't good tank mates.
I'll give you a definate NO on this one..It can work for a WHILE but eventually the Oscar will get brave and start attacking the Arowanna. An Aro from 22" or so ON....can handle any size Oscar but below that level the O can do a lot of damage. This is especially true if you have more than one Oscar in the tank...
i would think the awrowana would be fine if he is already 10 inches while your oscar is 5. i keep a tiger albino oscar and a silver arowana together and they r cool. but my guys r bigger than yours. oscar about 9-10 in, arowana is about 16in