Measure 1 cup old-fashioned rolled oats into a large mixing bowl. Pour 1 1/2 cups boiling water over the oats and let them sit until cooled.
Preheat oven to 350 degrees. Spray a 9x13 pan with non-stick baking spray, or grease and flour the pan.
In a stand mixer or mixing bowl with an electric mixer, blend 1/2 cup shortening,1 cup brown sugar, 1 cup granulated sugar, and 2 eggs. Continue to blend until the mixture is light and fluffy.
In a separate bowl, sift together the dry ingredients: 1 1/2 cups flour, 1 teaspoon cinnamon, 1 teaspoon baking soda, 1/2 teaspoon nutmeg, and 1/2 teaspoon salt.
Add the cooled oats and water and blend.
Add the dry ingredients to the mixer and blend well.
Pour the cake batter into the prepared 9x13 pan. Bake for 35 minutes or until a toothpick inserted into the center of the cake comes out clean. While the cake is baking, prepare the topping.