Combine butter and brown sugar in a large mixing bowl; stir in eggs, mixing well. Add flour, chopped orange slices, and pecans; stir until well blended.
Pour batter into a wellgreased 15- x 10- x 1-inch jellyroll pan. Bake at 350° for 25 minutes. Cool 15 minutes in pan; cut into 2- x 2-inch squares. Cool completely in pan. Remove bars from pan; cut in half diagonally, and roll in powdered sugar.