Preheat oven to 350 degrees. Grease 2 loaf pans, then line the bottom and sides with waxed paper.

Combine brown sugar and oil in a large bowl and beat well. In a small bowl, beat buttermilk, egg, vanilla, baking soda, and salt. Add to brown sugar mixture and blend thoroughly. Gently fold in the flour, rhubarb, and nuts. Divide batter evenly between prepared loaf pans.

To make topping, combine sugar, orange peel, and butter in a small bowl and mix well. Sprinkle on each bread.

Bake about one hour or until toothpick comes out clean. Let cool in pans 10 minutes, then turn out onto baking racks. Remove wax paper when the loaves are completely cooled.