Preheat oven to 350°F .
Sprinkle 1/3 cup sugar over the rhubarb and set aside.
In medium bowl pour boiling water over oats, cover and let stand 20 minutes. Meanwhile place 2 tbs margarine in 8x8x2 pan and heat in a 350 oven for 2 minutes. Stir in the sugar-sprinkled rhubarb.
Stir flour, baking powder, soda, cinnamon, and salt together. Set aside.
In large mixing bowl combine 2/3 cup sugar, brown sugar and egg, stirring until combined. Add the oatmeal and mix well. Add flour mixture to oat mixture, and beat until combined.
Carefully pour the batter over rhubarb mixture. Bake in preheated 350°F oven for 50 minutes or until a toothpick comes out clean. Invert onto a plate and let cool a few minutes, then enjoy.
Contributor: Roger Manley