In a 3-quart pot, preheat 4-inches oil to 375 degrees F.
In a shallow bowl, combine flours, cayenne, paprika, salt, and white pepper. Pour the buttermilk into another shallow dish.
Toss the onion rings in the flour mixture and shake off excess flour. Add to buttermilk mixture and make sure the rings are thoroughly coated. With a clean hand, add rings to flour mix again and shake off any excess flour.
Begin to fry in preheated oil in small batches, 1/4 of total amount at a time. Cook onion rings for about 3 to 5 minutes, turning occasionally with metal tongs or slotted spoon, until crispy and browned. Drain on paper towels and keep in warm place (low oven, about 200 degrees F) until ready to serve. Sprinkle with extra salt, if needed.