Combine oatmeal, flour, brown sugar, soda and salt. Cut in butter until crumbly. Lightly press 2/3 of this mixture into a 9 x 13 pan. Bake at 350 for 12 min.
In a saucepan combine cornstarch and sugar. Add water and rhubarb. Cook for about 5 min. stirring constantly. Pour over crust and sprinkle with the remaining topping and nuts. Continue baking for 25 to 30 more min. Serve with whipped cream or ice cream.