Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners or lightly grease with oil.
Whisk together all dry ingredients in a large bowl: oat flour, baking soda, baking powder, salt, cinnamon, pumpkin pie spice, and nutmeg until well combined.
In a separate bowl, combine the wet ingredients: pumpkin purée, eggs, maple syrup, melted coconut oil, vanilla extract, and milk. Whisk until smooth and no egg streaks remain.
Pour the wet mixture into the dry ingredients and gently fold together using a spatula until just combined. Be careful not to overmix – a few small lumps are perfectly fine.
Fold in any optional mix-ins like chocolate chips or nuts if using, with just a few strokes to distribute them evenly.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full.
Bake for 22-25 minutes or until a toothpick inserted in the center comes out clean or with just a few moist crumbs attached.
Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.