Preheat oven to 350 degrees. Add oats to a large bowl, pour water over oats and let sit until cooled.

In a separate bowl, cream brown and white sugar with eggs and oil. Sift together remaining ingredients and add to sugar mixture. Fold in oatmeal mixture.

Pour batter into greased 13×9 baking dish. Bake for 35-40 minutes or until cake tester inserted into the center comes out clean.

When there is 10 minutes remaining, begin making frosting. In a large sauce pan, combine margarine, brown sugar and evaporated milk.

Cook over medium heat, stirring constantly, until thickened, about 5-6 minutes. Once thick, remove from heat and add vanilla, coconut and walnuts.

Spread frosting over hot cake. Allow to completely before serving.