In a large mixing bowl, combine rolled oats, baking powder, ground cinnamon, salt, and brown sugar.

In a separate bowl, whisk together milk, vanilla extract, egg, and melted butter (or coconut oil).

Pour the wet ingredients into the dry ingredients and mix until fully combined.

Fold in the blueberries gently to avoid crushing them.

Lightly grease a 9x9-inch baking dish or similar-sized oven-safe dish.

Pour the oatmeal mixture into the prepared dish and spread it evenly.

Cover the dish with plastic wrap or foil and refrigerate overnight.

In the morning, preheat your oven to 375°F (190°C).

Remove the oatmeal dish from the refrigerator and uncover it.

If desired, sprinkle chopped nuts or shredded coconut over the top for added texture.

Bake the oatmeal for 35-40 minutes, or until the top is golden brown and the oatmeal is set.

Allow to cool for 5 minutes before serving. Serve warm and enjoy!