Combine onions and buttermilk in a large bowl; soak for 5 minutes.
Meanwhile, pour oil into a large pot to a depth of 1-1/2 inches; heat to 375 degrees.
Mix flour, cornmeal, cayenne, salt, and pepper in a medium bowl. Drain onion rings.
Working in small batches, add a few onion rings to flour mixture and turn to coat. Carefully add to oil. Fry until golden brown, 2 minutes. Drain on paper towel. Sprinkle with salt and pepper to taste.