In a large bowl, combine sugar, cornstarch, cinnamon and nutmeg; beat in sour cream and egg.
Gently fold in rhubarb.
Pour into pastry shell.
For the topping, combine oats, brown sugar, margarine, and flour; cut in the margarine until mixture is crumbly.
Sprinkle the topping over the filling.
Bake at 400F for 15 minutes. Reduce heat to 350F and bake for 35-40 minutes or until topping is golden brown.
Cool pie on a rack, serve warm** or cool.
**NOTE slicing this pie fresh out of the oven while still warm will be a little more difficult as the sour cream is still soft; if you want a nice slice that holds together, refrigerate the pie till cool then slice and heat a bit in the microwave if you would like it served warm.