Lightly spoon flour into dry measuring cups,; and level with a knife. Combine flour and the next 4 ingredients in a large bowl; make a well in the center. Combine bananas, buttermilk, oil, and egg in a bowl; add to flour mixture. Stir just until moist. Spoon batter into an 8 x 4-inch loaf pan coated with cooking spray.
Combine 1/4 cup oats and remaining ingredients and sprinkle over batter. Bake at 350 for 1 hour or until toothpick inserted in the center comes out clean. Cool 5 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.