Cut onions into 1/2-inch slices, and separate into rings. Set aside.
Whisk together flour and buttermilk until smooth. Pour oil to a depth of 2 inches into a Dutch oven; heat to 375°. Dip onion rings in batter, coating well.
Fry, a few rings at a time, until golden. Drain on paper towels. Season with salt to taste. Serve immediately.