Combine first 4 ingredients in a large bowl and beat well.
Combine banana, oats, and milk; add to sugar mixture, beating well.
Combine flour, baking powder, baking soda, salt, and cinnamon and stir with a whisk. Add to sugar mixture and beat just until moist.
Divide batter among greased muffin tins, and bake 15-20 minutes or until wooden pick inserted in center comes out clean. (This recipe may also be cooked in a 9x5-inch loaf pan for 1 hour and 10 minutes at 350F).