Preheat oven to 350 degrees F (175 degrees C). Grease 18 muffin cups or line with paper liners.
In a medium bowl, mix together oil, pumpkin puree, eggs, 1/2 cup brown sugar, and 1/2 cup white sugar until smooth.
In a separate bowl, sift together flour, baking soda, cinnamon, nutmeg, ginger, and salt. Stir the dry ingredients into the wet mixture until just combined.
Fold in the diced apples and walnuts. Divide the batter among the prepared muffin cups.
In a small bowl, mix together the oats, 1/4 cup brown sugar, and melted butter until crumbly. Sprinkle the streusel topping over the muffins.
Bake in the preheated oven for 20 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Interesting Facts
Pumpkin puree is a great addition to muffins, as it provides moistness and flavor.
Apples are a great addition to muffins, as they provide texture and sweetness.
Streusel is a classic topping for muffins, adding a crunchy texture and sweetness.