Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
In a large bowl, combine all dry ingredients – oats, baking soda, salt, and pumpkin pie spice. Whisk together to ensure even distribution of the spices and leavening agent.
Add all wet ingredients to the same bowl – pumpkin puree, eggs, maple syrup, milk, oil, and vanilla. Stir until just combined; avoid overmixing which can make your muffins tough.
Fold in your optional mix-ins gently with a spatula. Dark chocolate chips provide a delicious contrast to the earthy pumpkin flavor, while walnuts add wonderful texture.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full. The batter will be thick, so you may need to use a spoon or ice cream scoop to portion it out.
Bake for 18-22 minutes until the tops are set and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. This prevents the bottoms from getting soggy.