Heat the oven to 400 degrees. Cut the sweet potatoes lengthwise in 1-inch wedges. Arrange the wedges in a single layer in 10x15-inch nonstick baking pan with sides. Drizzle the potatoes with oil, turning to coat all sides. In a small bowl, mix the onion powder, paprika, salt, oregano, sugar and garlic powder. Sprinkle over the potato wedges. Bake for 15 minutes. Turn the wedges over and bake for an additional 5 to 10 minutes longer, or until tender. Arrange on a serving platter. Sprinkle with the green onions and garnish with the sour cream.