Oatmeal Currant Scones

• 1 2/3 cups all-purpose flour
• 1/4 cup plus 2 tablespoons sugar plus additional for sprinkling
• 1 tablespoon baking powder
• 3/4 teaspoon baking soda
• 1/2 teaspoon salt
• 1 1/3 cups old-fashioned oats
• 1 1/2 sticks (3/4 cup) cold unsalted butter, cut into tablespoon pieces
• Finely grated zest from 1 large navel orange
• 2/3 cup well-shaken buttermilk plus additional for brushing
• 1/2 cup dried currants
• Special equipment: a 2 1/4-inch round cookie cutter