Preheat oven to 350 degrees F (175 degrees C). Grease 12 muffin cups or line with paper muffin liners.
For muffin mixture, place the oats you have processed in the food processor in a large bowl, together with the flour, the rolled oats, chopped walnuts, baking powder, cinnamon and salt. In a separate bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the banana and vanilla. Stir in sour cream. Mix oat/flour mixture into egg mixture. Spoon batter into prepared muffin pans.
To make the topping: combine the oats, brown sugar and cinnamon in a small bowl. Cut in butter until mixture resembles coarse crumbs, then stir in the chopped walnuts. Generously sprinkle muffins with topping, and bake in preheated oven for 25 to 30 minutes, until golden brown and a toothpick inserted into a muffin comes out clean.