1. Rub a little vegetable oil on the skin of each sweet potato and bake them in a preheated 400 degree oven for 60 to 70 minutes or until they are tender. When the potatoes are cool enough to handle, scrape out the insides and use an electric mixer on high speed to beat the potatoes until they are mashed and smooth. Measure exactly 6 cups of the mashed sweet potato into a large bowl.
2. Add 3/4 cup brown sugar, heavy cream, melted butter, 1/4 teaspoon cinnamon, and salt into the sweet potatoes and mix well with the beater until all the ingredients are incorporated. Pour this mixture into an 8x8-inch baking dish.
3. Preheat oven to 350 degrees.
4. Make the oatmeal streusel by grinding the rolled oats to a fine flour using a food processor. A blender will also work.
5. Combine the oat flour with 4 tablespoons brown sugar, all-purpose flour, and 1/4 teaspoon cinnamon in a small bowl. Cut the cold butter into the dry mixture using a pastry knife or a fork. You should have a crumbly mixture with pea-size bits. Sprinkle this oatmeal streusel over the sweet potato mixture and pop it into the oven for 70 to 80 minutes or until the top begins to brown slightly.
6. When you remove the casserole from the oven immediately spread the marshmallows over the top. Let this sit for about 10 minutes. The heat from the casserole will melt the marshmallows, and then it's ready to serve.