In a 5- to 6-quart slow cooker, combine broth, carrots, dried oregano (if using),1/2 teaspoon salt, and 1/4 teaspoon pepper. Gently add meatballs.Cover and cook on low-heat setting for 6 hours or on high-heat setting for 3 hours, stirring in pasta and spinach during the last 20 minutes of cooking. Ladle into bowls. If desired, garnish with oregano sprigs. Soup will thicken upon standing.