Place onion rings into a bowl and cover with buttermilk.
Cover with plastic and place in the fridge for 1 hour (or up to 24 hours).
Stir flour, salt, paprika, garlic powder, and cayenne pepper in another large bowl.
Add the onion to the flour mixture and toss to coat. If you prefer extra crispy onion rings, do a double dredge (add the coated rings back into the buttermilk then dredge in flour again).
Place oil in a large, heavy saucepan to a depth of about 3 inches, or heat your deep fryer, to 365° F.
Working in smaller batches, shake excess flour off onions, and cook in hot oil until golden brown and crispy, about 2 minutes per batch.
Transfer onions to paper towels.
Season with a little more salt (if desired) and serve immediately! Delicious served with Kickin' Remoulade!