Preheat your oven to 350°F (175°C).
Prepare your muffin tin by greasing it or lining it with paper cups.
In a large bowl, combine the dry ingredients: flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
In another bowl, whisk together the granulated sugar, brown sugar, eggs, oil, pumpkin puree, and vanilla extract.
Gradually add the dry mixture to the wet ingredients, stirring until just combined.
Fold in the chopped walnuts gently.
In a separate bowl, prepare the streusel topping by combining oats, flour, and cold butter.
Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.
Sprinkle the streusel topping evenly over the muffin batter.
Bake the muffins for 20-25 minutes or until a toothpick comes out clean.
Remove the muffins from the oven and allow to cool in the pan for 5 minutes before transferring to a wire rack.
Enjoy warm or at room temperature, and consider pairing with a cup of coffee or tea.