Mix flour, salt, and 1/2 teaspoon of cinnamon into a large mixing bowl. Add sugar and oil and mix on medium speed until well blended and feathery.
Remove 3/4 cup of this mixture and put in a smaller mixing bowl for the topping. Add walnuts and remainder of cinnamon to the contents of the smaller bowl for the topping, mix well and set aside for later.
To remaining mixture (not the topping) add baking soda, baking powder, egg and buttermilk. Mix until smooth, this is the coffeecake batter.
Spoon the cake batter into a buttered 9 x 13 baking dish. Sprinkle the topping over the cake batter and press lightly with fingers. Bake at 350 degrees for 25 - 30 minutes.