In a large bowl, beat the eggs, buttermilk, brown sugar and oil. Combine the flour, baking powder, baking soda, cinnamon and salt; gradually add to batter until blended. Fold in the oats, blueberries and walnuts if desired.
Transfer to a greased and floured 10-in. fluted tube pan. Bake at 375° for 45-50 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely. Dust with confectioners' sugar.