Preheat oven to 400°F. Lightly spray or grease a muffin tin with oil. Set aside. In a large bowl, combine oats and buttermilk. Let sit 15 minutes.
In a second mixing bowl, whisk flour, baking soda, baking powder, salt and cinnamon.
Stir the oil, egg, brown sugar and vanilla into the soaked oat mixture. Add dry ingredients. Stir just until combined. Do not overmix. Gently fold apples into the batter.
Spoon the batter into the prepared muffin tin. Bake 12 minutes. Remove from the oven. Let the muffins cool in the pan 5 minutes. Transfer to a cooling rack.
TIP: These muffins are best served warm. They also freeze and reheat incredibly well. To enjoy after freezing, thaw at room temperature or heat frozen muffins in a 350F oven for 10-15 minutes. NOTE: I don’t recommend using paper muffin cups in this recipe. Baking directly on the muffin tin produced the best results – keeping the moisture in and creating a nice crust on the outside.