How to Make It
Preheat oven to 350°F. Stir buttermilk into 1/2 cup oats in a bowl; let stand 10 minutes.
Place 1 1/2 cups flour in a bowl. Stir in baking powder, baking soda, and 1/2 teaspoon salt.
Combine 3 tablespoons sugar, 2 tablespoons oil, nuts, cinnamon, 1/8 teaspoon salt, and remaining oats and flour in a small bowl.
Add bananas, vanilla, eggs, and remaining brown sugar and oil to buttermilk mixture; fold in flour mixture. Spoon batter into a 9- x 5-inch loaf pan coated with cooking spray. Sprinkle walnut streusel over top. Bake at 350°F until a wooden pick inserted in center comes out clean, 55 minutes to 1 hour. Cool in pan on a wire rack 15 minutes. Remove bread from pan; serve warm, or cool completely.