In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, milk, egg, garlic, salt, pepper, oregano, basil, onion powder, and parsley. Mix until well combined.
Form the mixture into evenly sized meatballs, about 1 to 1.5 inches in diameter.
Heat olive oil in a large skillet over medium heat. Add meatballs and cook until browned on all sides, about 5-7 minutes.
Reduce heat and add marinara sauce (if using). Cover and let simmer for an additional 10-15 minutes until the meatballs are fully cooked.
Serve warm over pasta, in a sandwich, or on their own with extra sauce.