Combine rolled oats and buttermilk in a large bowl.

Let the mixture stand for one hour (or 30 minutes if in a rush).

Preheat oven to 400 degrees F (200 degrees C).

Line a muffin tin with 10 paper muffin cups.

Stir the egg and brown sugar into the oat mixture.

Add the melted and cooled butter to the oat mixture and stir to combine.

Combine flour, salt, baking powder, baking soda, cinnamon, and nutmeg in a separate bowl.

Add the dry ingredients to the wet ingredients and stir until just combined.

Gently fold in raisins or chocolate chips (if using).

Divide the batter evenly among the prepared muffin cups.

Bake in the preheated oven for about 25 minutes, or until the muffins are firm and golden brown.

Let cool in the pan for a few minutes.

Transfer the muffins to a wire rack to cool completely.