Preheat the oven to 350ºF. Lightly oil three mini loaf pans. In a medium bowl, combine the oat flour, baking powder, baking soda, cinnamon, and salt, and whisk together to combine. In a larger bowl, mash the bananas with a fork or potato masher, then whisk in the sugar, vanilla extract, and oil until well-combined. Stir in the oat flour mixture and mix well. Fold in the walnuts.
Divide the batter evenly between the mini loaf pans. Mix together the rolled oats and turbinado sugar for the Sugared Oat Topping, and sprinkle evenly over the loaves, pressing gently into the batter to help the oats stick. Place the loaves on a baking sheet, transfer to the oven, and bake for 25 to 30 minutes, until loaves feel firm when touched. Let cool on a cooling rack for 10 minutes before gently removing from pans. Allow to cool completely before slicing and eating.
Store any leftover slices in an airtight container at room temperature for up to 5 days.