Preheat your oven to 375°F (190°C) and line a muffin tray with liners or lightly grease it.
In a bowl, combine oats and milk. Let sit for 10 minutes to soak.
Add the egg, oil, and brown sugar to the soaked oats. Mix well.
In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
Gradually add the dry ingredients to the wet ingredients. Mix until just combined.
Divide the batter evenly among the muffin cups.
Bake for 18–20 minutes or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.