Preheat oven to 400°. Mix together flour, oatmeal, baking powder and soda, and cinnamon.
Stir together applesauce, brown sugar, melted margarine, egg and vanilla extract. Stir wet ingredients into dried ingredients.
Spoon batter into paper-lined muffin cups and mix streusel topping.
To make Streusel: in a bowl combine flour, sugar, and cinnamon. Cut in butter until crumbly. Stir in walnuts.
Sprinkle streusel on top of muffins. Place muffins in oven and bake for 18-20 min for smaller muffins, 20-25 for larger muffins. Set to cool on a wire rack.