Combine buttermilk, flour and salt in a large bowl until smooth. Soak the onion rings in the batter for 2 hours in the refrigerator. Heat oil in the Dutch Oven to 375 degrees F. In batches, remove onion rings from batter and dredge in the seasoned flour mixture. Fry for 3 to 4 minutes or until golden brown. Drain on paper towels and season immediately with salt.