Preheat the oven to 325 degrees F. Grease a 9x5 inch loaf pan. Line the pan with a strip of parchment, then dust with flour.
In a large bowl, whisk together the oat flour, flour, baking powder, baking soda, salt and cinnamon; set aside.
Put the bananas in a mixing bowl and beat them with your electric mixer until they are fully mashed. Pour the bananas into a small bowl and set aside.
In the mixing bowl, beat the butter and sugar until creamy. Add the eggs, one at a time, beating until fluffy and scraping bowl often. Beat in the vanilla and the milk.
Fold the flour mixture into the batter until it is incorporated. Do not overbeat.
Stir in the mashed bananas, then stir in the nuts (if using).
Spread the batter evenly in the pan and bake for 1 hour or until a toothpick inserted comes out clean.
Let cool in the pan for about 20 minutes, then carefully remove from pan.