Heat oven to 400°F. Line twelve medium muffin cups with paper baking cups or spray bottoms only with cooking spray. For streusel, in small bowl, combine oats, flour and sugar; mix well. Cut in butter with pastry blender or two knives until mixture is crumbly. Set aside. For muffins, in large bowl, combine flour, oats, sugar and baking powder; mix well. In small bowl, combine milk, oil, egg and vanilla; 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 18 to 20 minutes or until golden brown. Cool muffins in pan on wire rack 5 minutes. Remove from pan. Serve warm.