In a medium bowl pour the boiling water over the rolled oats; cover and let stand for 20 minutes. Meanwhile, place the margarine or butter in an 8x8x2-inch baking pan and heat in a 350 degree F oven about 2 minutes or until melted; stir in the 1/3 cup granulated sugar. Sprinkle the diced rhubarb over the sugar; set aside.
In a medium bowl stir together the flour, baking powder, baking soda, cinnamon, and salt; set aside. In a large mixing bowl combine the 2/3 cup granulated sugar, brown sugar, oil, and egg; beat until combined. Add the oat mixture and beat well. Add the flour mixture to the oat mixture, beating just until combined.
Carefully pour the cake batter atop the rhubarb mixture. Bake in a 350 degree F oven about 50 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack for 5 minutes. Run a knife along sides of pan to loosen cake; invert onto a serving plate. Serve warm. Makes 9 servings.
If fresh rhubarb is unavailable, measure 2 cups frozen sliced rhubarb; thaw slightly and chop. Return to bowl and thaw completely before using. One pound of fresh rhubarb yields 3 cups sliced rhubarb.