Dissolve soda in buttermilk; stir well. Combine cornmeal, baking powder, and salt. Stir in buttermilk mixture, egg, and onion, mixing well.
Carefully drop batter by tablespoonfuls into deep, hot oil (350°), cooking only a few at a time. Fry for 3 minutes or until hush puppies are golden brown, turning once. Drain well on paper towels.