Preheat oven to 275 degrees; set a rack on a baking sheet, and place in oven. In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt; set aside. In a large bowl, whisk together buttermilk, butter, and eggs; add flour mixture, and mix just until batter is combined.
Heat waffle iron according to manufacturer's instructions; brush with oil. Pour batter onto iron (amount depends on iron size), leaving a 1/2-inch border on all sides (spread batter if necessary). Close iron; cook until waffles are golden brown and crisp, 3 to 5 minutes. Transfer to rack in oven to keep warm; repeat with remaining batter.
Like pancakes, waffles require a gentle hand when preparing the batter. For the most tender results, don't overmix it -- there should still be some lumps.