1. Preheat the oven to 350 degrees F. Line two muffin tins with paper liners or spray with cooking spray. Set aside.
2. In a medium bowl, whisk together flour, baking soda, salt, cinnamon, and nutmeg. Set aside.
3. In a large bowl, whisk together sugar, oil, water, pumpkin, eggs, and vanilla extract. Whisk until smooth and combined.
4. Slowly stir in the flour mixture. Mix until ingredients are combined.
5. For the brown sugar cinnamon filling: in a small bowl, mix the brown sugar and cinnamon together. Set aside.
6. For the streusel topping: in a small bowl mix together the oats, flour, brown sugar, cinnamon, and nutmeg. Mix in the butter with your hands until the mixture is crumbly.
7. Fill muffin cups half way full with the pumpkin batter. Sprinkle the brown sugar cinnamon filling over each half filled muffin cup. Fill the muffins cups with the remaining batter. Top each muffin with streusel topping.
8. Bake for 17-20 minutes or until a toothpick comes out clean. Remove pans from oven and cool on a wire rack. Remove muffins from tins and enjoy.