I would just do the usual routine for something like this, keep the water clean and keep the environment as stress-free as possible for the fish. it's good that you've separated the fish from the aggressor. I would keep it separated until the Arowana puts more size over the oscar, as a 5-inch oscar is a lot more fish than a 5-inch Arowana. Your main concern now should be preventing any secondary bacterial infections, since a shredded gill plate and fin are survivable for an Arowana. I would keep some anti-bacterial meds on hand to be used as soon as something seems strange with the fish. Other than that, it should heal up fine given that your water quality is pristine. I would hold off on feeding for a few days, until the fish's condition improves, and try to feed with something that would be more likely to elicit a feeding response, like bloodworms instead of pellets.