In a small bowl, combine oatmeal, brown sugar, margarine, flour & citrus zest. Cut in margarine until mixture is crumbly. Set aside.
Preheat oven to 400 F. In a large bowl, combine sugar, cornstarch, cinnamon & nutmeg; beat in sour cream & egg. Gently fold in rhubarb. Pour into pastry shell. Sprinkle topping mixture over the filling.
Bake at 400 F. for 15 minutes. Reduce heat to 350 F. & bake for 35-40 minutes or until topping is golden brown. Cool on a wire rack before slicing.
In order to obtain nice slices, refrigerate pie until cold then slice & heat a bit in the microwave if preferred.