In a large Dutch oven, pour oil to a depth of 4 inches, and heat over medium-high heat until a deep-fry thermometer registers 360°.
In a blender, pulse rice until finely ground. In a shallow dish, combine rice, flour, salt, onion powder, and cayenne. In another shallow dish, place buttermilk. Dip fish in buttermilk, letting excess drip off . Dredge each fillet in rice mixture, pressing mixture to fish, and shaking off excess.
Carefully place fish into hot oil in batches. Cook about 3 minutes or until golden brown, turning occasionally. Remove with a slotted spoon; drain on paper towels. Serves 6.