Toss onions with buttermilk in large bowl. In another large bowl, whisk flour, salt, pepper, and cayenne pepper. Sprinkle 1/2 cup flour mixture on a rimmed baking sheet, set aside.

In a 5 qt sauce pan, heat oil to 325-350. Lift onion rings out of buttermilk (let excess drip off). Toss with remaining flour mixture to coat, place on floured sheet.

Work in small batches, carefully place coated onion rings (shake off excess flour) into hot oil, cook until golden brown, turning with tongs 2 to 3 min. Place on paper towels.