Heat oven to 375°F. For crust, combine flour, oats, sugar and salt in medium bowl; mix well. Add margarine, water and vanilla; mix well. Press mixture firmly onto bottom and sides of 9-inch glass pie plate, forming 1/4-inch rim around edge. Bake 12 to 15 minutes or until light golden brown. Cool completely on wire rack. For filling, combine sour cream, sugar, flour, egg whites, cinnamon and nutmeg in medium bowl; mix well. Add apples; mix well. Spoon into cooled crust. For topping, combine oats, sugar and flour in medium bowl; mix well. Cut in margarine with pastry blender or two knives until mixture is crumbly. Sprinkle over filling. Bake 50 to 60 minutes or until knife inserted in center comes out clean. Cool on wire rack. Serve warm or chilled. Store tightly covered in refrigerator.