Preheat oven to 375° F. Rinse the sweet potatoes and prick their skins several times with a fork. Place them directly on rack in oven. Bake 1 hour or until soft; set aside. Cook the rigatoni according to the package directions. Drain, reserving 1/3 cup of the cooking water. In a small skillet, heat the olive oil over medium-low heat. Add the garlic and sauté until just golden. Add the oregano and sauté about 1 minute, until fragrant and slightly crisp. Place the rigatoni in a large bowl. Scoop the potato flesh into the bowl of pasta. Add the olive oil mixture and pasta water and toss. Sprinkle with the salt, pepper, and Parmesan.