Mix flour, oatmeal, brown sugar, melted butter and cinnamon until crumbly. Press half of mixture into greased 9" baking pan. Cover crumb mixture with diced rhubarb. Combine sugar, cornstarch, water and vanilla. Cook until thick. Pour this sauce over rhubarb. Top with remaining crumbs and bake at 350 degrees for 1 hour. Serve warm, plain or topped with Cool Whip.

I usually double this recipe and use a 9 X 13" pan.