Preheat your oven to 400 degrees and line your muffin pan with cupcake liners. Set these aside and move on to making the muffins.
First, in a medium sized bowl, stir together the flour, oatmeal, baking powder, salt, baking soda, cinnamon, and nutmeg.
In a separate, smaller bowl, stir together the mashed sweet potatoes, brown sugar, oil, milk, egg, and vanilla until fully combined.
Add the sweet potato mixture to the bowl containing the dry ingredients. Stir with a wooden spoon just until all of the dry ingredients are moistened. Do not over mix the muffins or they will turn out tough.
Spoon the muffin batter into the prepared pan. Fill the muffin cups about halfway full.
Now, mix up the crumble topping. In a small bowl, stir together the flour, brown sugar and oats. Drizzle the melted butter over all of these and mix them well with a fork. Sprinkle the oat mixture over the top of the muffins, evenly dividing it among the muffins.
Slide the pan into the preheated oven and bake for 15 to 20 minutes. Let the muffins cool in the pan for about 5 minutes before you remove them and place them on a wire rack to completely cool.
Store the muffins in an airtight container for up to three days or in the freezer for up to a month.