Lightly spoon flour into dry measuring cups, and level with a knife. Combine flour and next 4 ingredients (flour through salt) in a large bowl; make a well in center of mixture. Combine banana, buttermilk, oil, and egg substitute 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, walnuts, 2 tablespoons brown sugar, and butter, and sprinkle over batter. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool 5 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.