Preheat oven to 375°F. Spray a casserole pan or glass baking dish with cooking spray.
In a medium bowl, mix together the ground beef, oats, egg, milk, Worcestershire sauce, and onion. Shape into small meatballs (1 - 1 ½ inches). Place in prepared baking dish.
In a small bowl, whisk together brown sugar, ketchup, Worcestershire sauce, mustard, and vinegar. Pour sauce over meatballs.
Cover with foil and bake for 45-50 minutes or until meatballs are no longer pink inside.
Sprinkle with parsley and serve over rice or mashed potatoes.