Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and the next 4 ingredients (through salt) in a large bowl, stirring well with a whisk. Stir in oats.
Combine mashed banana, buttermilk, vegetable oil, vanilla extract, and eggs; add to flour mixture. Stir just until moist.
Spoon batter into an 8 x 4-inch loaf pan coated with cooking spray. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool 15 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.