Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly.
In a large bowl, mix together the rolled oats, flour, baking powder, baking soda, cinnamon, nutmeg, ginger, and salt.
In another bowl, whisk together the pumpkin puree, maple syrup, vegetable oil, milk, egg, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Fold in the chopped walnuts if using.
Divide the batter evenly among the muffin cups, filling each about 3/4 full.
Bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.