Whisk together eggs, milk, flour, baking powder, and onion salt in a bowl to make a smooth batter.
Heat the oil in a large saucepan or deep fryer until a thermometer reads 350 degrees F (175 degrees C).
Dip the onion rings evenly into the batter, then fry in the oil for 2 to 3 minutes, working in batches if necessary, until the onion rings are golden brown. With a slotted spoon, remove the onion rings to paper towels to drain. Serve hot.