Preheat oven to 350°.
Combine first 4 ingredients in a large bowl; beat well at medium speed of a mixer. Combine banana, oats, and milk; add to sugar mixture, beating well. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, salt, and cinnamon; stir with a whisk. Add to sugar mixture; beat just until moist. Spoon batter into a 9 x 5-inch loaf pan coated with cooking spray. Bake at 350° for 1 hour and 10 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.