Gather all your ingredients and equipment so that you have everything close at hand on the counter or table.
Prepare the Onions
Using a knife and cutting board, slice onions 1/2" thick.
In large bowl, combine beer and vinegar. Add onion to brine and refrigerate for 30-60 minutes.
Prepare Dipping Sauce
In a small bowl, mix together the mayonnaise, ketchup, horseradish, smoked paprika, seasoned salt, oregano, and black pepper. Cover and refrigerate until time to serve.
Get Ready to Fry
Line a baking sheet with several layers of paper towels. Add peanut oil to a Dutch oven and heat over medium high to 350 degrees.
In a large bowl combine the all-purpose flour, cornstarch, baking soda, seasoned salt, and freshly ground black pepper. Drain beer mixture into bowl and whisk together until batter falls from whisk in a steady stream.
Pat onion rings dry with paper towels. Transfer 1/3 of rings to beer batter. Dip onion in the batter to cover.
Carefully transfer rings to hot oil. Fry until rings are golden brown and crisp, about 2 minutes, flipping halfway through frying. Drain rings on prepared baking sheet. Season with more seasoned salt and black pepper. Keep warm in oven. Return oil to 350 degrees and repeat until all rings are cooked.
Serve warm with the prepared dipping sauce. Serves 4-6.