Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a large mixing bowl, combine pumpkin puree, granulated sugar, brown sugar, eggs, vegetable oil, and vanilla extract. Mix until smooth and well combined.
In another bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the pumpkin mixture, stirring gently until just combined. Fold in the chopped pecans, if using.
Pour the batter into the prepared baking dish and spread it evenly.
For the streusel, in a separate bowl, mix together flour, brown sugar, rolled oats, and cinnamon. Pour in the melted butter and mix until crumbly.
Sprinkle the streusel topping evenly over the pumpkin batter.
Bake in the preheated oven for 45-55 minutes, or until a toothpick inserted in the center comes out clean.
Once done, remove from the oven and let it cool in the pan on a cooling rack for at least 15 minutes before serving.
Serve warm or at room temperature, optionally topped with whipped cream or vanilla ice cream.