1/2 cup plus 2 tablespoons sugar, divided
1 1/2 teaspoon ground cinnamon, divided
Vegetable oil (for frying; about 4 cups)
1 cup gluten-free old-fashioned oats
1/2 cup rice flour
2 tablespoons cornstarch
1 teaspoon baking powder
3/4 teaspoons kosher salt
1 large egg
1/2 cup (or more) club soda
2 large crisp apples, peeled, cored with an apple corer, sliced into 1/4"-thick rings
Whisk 1/2 cup sugar and 1 teaspoon cinnamon in a shallow bowl; set aside.
Fit a large pot with thermometer and pour in oil to measure 3". Heat over medium-high heat until thermometer registers 375°F.
Meanwhile, pulse oats in a food processor to a coarse powder. Transfer to a large bowl and whisk in rice flour, cornstarch, baking powder, salt, and remaining 2 tablespoons sugar and 1/2 teaspoon cinnamon. Whisk in egg and 1/2 cup club soda, adding more soda by the tablespoonful until the consistency of pancake batter.
Working in batches and maintaining oil temperature, dip apple rings in batter and fry, turning occasionally, until golden brown and crisp, about 4 minutes.
Transfer fritters to a paper towel–lined plate; let drain briefly, then toss in reserved cinnamon sugar.