Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan.
Prepare the Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Mix the Wet Ingredients: In a large bowl, combine the granulated sugar, brown sugar, pumpkin puree, vegetable oil, vanilla extract, and eggs. Mix until well blended.
Combine Mixtures: Gradually add the dry ingredient mixture to the wet ingredients, stirring until just combined. Do not overmix.
Make the Streusel Topping: In another bowl, mix the flour, oats, brown sugar, and cinnamon. Add the cold butter and use a pastry cutter or your fingers to mix until crumbly.
Assemble the Cake: Pour half of the pumpkin batter into the prepared baking pan. Sprinkle half of the streusel topping over the batter. Add the remaining batter on top and finish with the rest of the streusel.
Bake: Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve: Allow the cake to cool in the pan for about 10 minutes before slicing. Serve warm or at room temperature.