Stir together oats and buttermilk in a large mixing bowl. Let sit for 15 minutes.
Preheat oven to 350 degrees while oats are soaking.
Whisk the egg, vanilla, brown sugar, and oil into the oat mixture.
Dump flour into the wet ingredients and make a small well. Pour the baking powder, soda, and salt into the well and stir them into the flour with a measuring spoon.
Stir the batter all together just until moistened.
Spoon into prepared muffin cups. Sprinkle tops with oats if desired.
Bake at 350° for 15-18 minutes. Let cool in pan for about 5 minutes before removing to cooling racks.
Notes
We like to add 1/2-3/4 a cup of dried fruit and/or nuts to the batter before adding the dry ingredients.
Without optional ingredients, you get 10 muffins. With added fruit and nuts you get 12 muffins.