I've seen oscar / ray tanks, and I've never heard of anyone having a problem (except, maybe, for an undersized oscar or 2 going missing occasionally). That seems like a good sized tank for 2 rays, although I would just stay aware that Oscars are messy eaters.
Arowanas look gorgeous with stingrays, and it is something that I wholeheartedly recommend; the only thing better, maybe, would be more rays.
Best of luck!