Preheat oven to 350 degrees F. Line twelve 2 1/2-inch muffin cups with paper bake cups; coat paper cups with cooking spray. Or coat muffin cups with cooking spray.

Place 3/4 cup of the oats in a food processor; cover and process until ground. Transfer to a large bowl. Stir in another 3/4 cup of the oats, whole-wheat flour, all-purpose flour, brown sugar, baking powder, baking soda and salt. Make a well in center of flour mixture.

In a medium bowl combine buttermilk, eggs, oil and vanilla. Stir in rhubarb. Add rhubarb mixture all at once to flour mixture; stir just until moistened (batter should be slightly lumpy). Spoon into prepared muffin cups, filling each about three-fourths full.

For streusel topping, in a small bowl stir together brown sugar and cinnamon. Stir in the remaining 1/4 cup oats and walnuts. Sprinkle over batter in muffin cups.

Bake 20 to 22 minutes or until a toothpick comes out clean. Cool in muffin cups on a wire rack for 5 minutes. Remove from muffin cups. Serve warm.