In a medium bowl, sift together the flour and salt; stir in oats and 1/2 cup sugar. Cut in shortening until mixture is crumbly. Pat half of the mixture into a 9 inch pie pan; set aside remaining half for the topping. Arrange rhubarb in pie shell. Sprinkle 1/2 cup sugar, cinnamon and water over rhubarb, then dot with butter. Spread remaining oat mixture over filling.
Bake for 40 minutes in the preheated oven, until rhubarb is tender.