Preheat your oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a bowl, combine the oats and milk. Let them sit for about 10 minutes.
In another bowl, whisk together the flour, brown sugar, baking powder, baking soda, salt, and cinnamon.
Add the egg, oil, and vanilla to the oat mixture and mix well.
Combine the wet and dry ingredients, mixing just until moistened.
Fill the muffin cups about ⅔ full with the batter.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for a few minutes before transferring to a wire rack to cool completely.