Preheat oven to 350F. Grease an 8 x 4 inch loaf pan. Set aside.
Add the banana to a large bowl and mash.
1 cup mashed banana
Stir in buttermilk until well-combined.
1/2 cup buttermilk
Whisk in the oil and vanilla until combined.
1/4 cup vegetable oil, 1 teaspoon vanilla
Add the eggs and sugars. Whisk in until combined.
2 large eggs, 1/3 cup granulated sugar, 1/3 cup light brown sugar
Add flour, baking powder, baking soda, salt, cinnamon, and oats. Whisk in just until combined. Don't overmix.
1 ½ cups all-purpose flour, 1 ½ teaspoons baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon salt, 1/2 teaspoon ground cinnamon, 3/4 cup old-fashioned oats
Pour batter into the prepared pan.
Bake for 50-60 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for about 10 minutes.
Remove to a wire rack to finish cooling.
Notes
Flour: To properly measure the flour, weigh it (you can find the weights by clicking "Metric" above the ingredients) or stir or sift the flour, lightly spoon into the measuring cup, and level.
Baking: If the bread is browning too quickly, tent with aluminum foil.