In a medium bowl, combine 1/2 cup sugar and cinnamon. In another medium bowl, whisk buttermilk, 4 teaspoons oil, and eggs. Stir in flour, 2 tablespoons sugar, baking powder, and salt.
In a large, heavy, high-sided skillet, heat 2 cups oil until it registers 375 degrees on a deep-fat or candy thermometer. (The oil should bubble rapidly when a drop of batter is added.) In batches, coat apples in batter and fry until golden brown, about 4 minutes per batch, flipping halfway through. Transfer to a paper-towel-lined baking sheet to drain. Toss apples in cinnamon sugar and serve immediately.