Preheat the oven to 350 degrees and grease a 8½ x 4½ inch loaf pan.
In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon.
Break the bananas into pieces and place them in the large bowl of a stand mixer. Beginning at a slow speed and gradually increasing, whisk the bananas until they become mashed. Remove the bananas to a smaller bowl and set aside.
Using the same bowl of the stand mixer, add the egg, brown sugar, oat milk, avocado oil, and vanilla extract. Mix to combine. Then add the mashed banana and mix thoroughly. Finally, add the dry ingredients and mix until just combined.
Transfer the batter to the prepared loaf pan, and bake for 55 to 60 minutes, or until the bread is set and a toothpick inserted in the center of the loaf comes out clean. (Note: Check the bread after 45 minutes in the oven, and loosely cover the loaf with foil if it’s beginning to get too dark.)
Cool the bread on a wire rack until it reaches room temperature, then store in an airtight container for up to 3 days. Otherwise, transfer to the freezer to enjoy another time.