If you don't have oyster sauce, you can sub 1 tablespoon soy sauce + 1 tablespoon brown sugar. It won't have quite as much umami flavor, but will do in a pinch.
Be sure to cut flank steak AGAINST the grain. See which direction the muscle fibers are running, then slice across it.
Nutritional information is an estimate for your convenience. If you have strict nutritional needs, please do your due diligence to make sure this recipe fits your needs.