Make 9"x13" cake as directed on box (Betty Crocker French Vanilla is perfect). Add 1 t cinnamon to batter and an additional 1/2 c flour to thicken batter so streusel doesn't sink to bottom.
Combine streusel ingredients. Use your fingers and/or a fork to incorporate butter until mixture looks like tiny peas. You want small clumps of butter so don't over mix. Scatter streusel over top of cake batter. Top with additional oatmeal. Bake for 25-30 minutes at 350 degrees. Use toothpick to test cake doneness. Cool for about 20 minutes. Drizzle frosting over top. Delicious served warm!