Step 1 Heat the oven to 350 degrees. Grease a 13x9-inch baking pan.
Step 2 Bring the water to a boil in a saucepan. Add the oatmeal, reduce the heat and cook until the water is absorbed, about 5 minutes. Remove the oatmeal from the heat and set aside. Allow it to cool to room temperature.
Step 3Cream the butter in a large mixing bowl and gradually beat in the granulated and brown sugars. Blend in the eggs and vanilla and beat for 2 minutes. Add the oatmeal.
Step 4Mix the flour, baking soda, salt, cinnamon and nutmeg in a separate bowl. Gradually add the dry ingredients to the sugar-butter mixture and blend in. Mix for 2 minutes.
Step 5Pour the batter in the pan. Bake the cake until a toothpick inserted in the center comes out clean, about 45 minutes. Allow the cake to cool before frosting.