Preheat oven to 350 degrees F. In a skillet heat olive oil. Add carrot, celery and cook for 3 minutes. Then add mushrooms, garlic, oregano and cook, stirring for 3 minutes more. Stir in spaghetti sauce. Arrange manicotti tubes in a 9 x 13-inch baking dish. Spoon sauce over manicotti and sprinkle with Parmesan cheese. Place in oven and cook for 20 minutes. Uncover and bake for 5 to 10 minutes more until bubbling and heated through. Serve each portion topped with fresh parsley.