Preheat your oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a mixing bowl, combine the oats and milk. Let sit for 10 minutes.
In another bowl, whisk together the flour, brown sugar, baking powder, baking soda, and salt.
Add the egg, oil, and vanilla extract to the oat mixture. Mix well.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Spoon the batter into the prepared muffin tin, filling each cup about ⅔ full.
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, then transfer to a wire rack to cool completely.