In a large bowl, combine oats, flour, brown sugar, baking soda and salt. Cut in butter until mixture resembles coarse crumbs. Press into a greased 13-in. x 9-in. x 2-in. baking pan. In a saucepan, combine dates, sugar and water. Cook for 10 minutes or until thickened, stirring frequently. Stir in walnuts. Spread over crust. Bake at 350 degrees F for 30 minutes. Cool on a wire rack. Cut into squares; top with whipped topping.