Preheat oven to 350.In a large mixing bowl beat butter, oil, cocoa, and sugar until smooth. Add pumpkin, eggs, and salt and mix again. Slowly add in flour and mix until combined. Stir in oreos and marshmallows. Spread into a greased 9x13" pan. Bake for 20-25 minutes or until a toothpick inserted into the center of the cake comes out clean.
Let the cake cool about 10 minutes, then poke holes all over the cake and pour chocolate syrup evenly over the cake. Let cool completely. Spread cool whip on top of the cake and sprinkle the chopped oreos on top. Store covered in the refrigerator.