In a medium bowl, mix together ground beef, bread crumbs, onion, Parmesan cheese, milk, Worcestershire sauce, salt, pepper, oregano, and garlic. Shape into 12-15 meatballs and place in a lightly greased crock pot.
Pour spaghetti sauce over meatballs. Cover and cook on LOW for 6 hours.
Split hoagie rolls open. Stuff with meatballs and sauce. Top with 2 slices cheese and broil until cheese is melted.