Preheat broiler. Coat a large nonstick skillet with nonstick spray; preheat over medium heat. Add mushrooms, onion, and garlic to hot skillet; cook for 5 to 10 minutes or until tender, stirring occasionally. Add tomato sauce, balsamic vinegar, rosemary, and the dried oregano. Bring to boiling; reduce heat. Simmer, covered, for 2 minutes. Stir in meatballs. Cook about 5 minutes more or until meatballs are heated through.
Meanwhile, open buns so they lay flat and place on a baking sheet. Broil 4 to 5 inches from the heat about 1 minute or until lightly toasted. Divide meatball mixture among buns. Sprinkle with cheese. Broil for 1 to 2 minutes more or until cheese is melted. If desired, sprinkle with fresh oregano.