Mix 1 1/2 cups sugar and 4 Tbsp cornstarch. Add 1/4 cup water and 1 tsp vanilla. Add 4 cups chopped rhubarb. Cook in saucepan on med-high heat until thick, stir often.
Mix 1 1/2 cups oatmeal, 1 1/2 cups flour, 1 cup brown sugar, 1/2 tsp soda, 1 cup shortening or butter and 1/2 cup chopped nuts until crumbly. Reserve 1 cup for topping. Press the rest into a 9x13 pan. Pour filling on top and sprinkle on the topping.
Bake at 350 for 30-35 minutes. Cut while slightly warm.