Heat oven to 400°F. Line twelve medium muffin cups with paper baking cups or spray bottoms only with cooking spray. For streusel, combine all ingredients in small bowl; mix well. Set aside. For muffins, combine flour, oats, sugar, nuts, if desired, baking powder, pumpkin pie spice, baking soda and salt in large bowl; mix well. In medium bowl, combine pumpkin, milk, oil and egg; blend well. Add to dry ingredients all at once; stir just until dry ingredients are moistened. (Do not overmix.) Fill muffin cups almost full. Sprinkle with reserved streusel, patting gently. Bake 22 to 25 minutes or until golden brown. Cool muffins in pan on wire rack 5 minutes. Remove from pan. Serve warm.