Fill a large bowl with room-temperature water and stir in the kosher salt until fully dissolved.
Add the chicken breasts to the brine and soak for 30 minutes to 2 hours.
After soaking, remove the chicken from the brine and pat dry with paper towels.
If necessary, pound the thicker end of the chicken breasts for even thickness.
Drizzle the chicken with olive oil and toss to coat evenly.
Combine the garlic salt, black pepper, onion powder, paprika, and oregano in a small bowl.
Sprinkle half of the spice mixture on the chicken, rubbing it in.
Spray the air fryer basket with non-stick cooking spray.
Preheat the air fryer for a few minutes.
Place two chicken breasts in the air fryer, set it to 380°F, and cook for 8 minutes.
Flip the chicken, add the remaining spice mixture, and cook for another 8-9 minutes.
Ensure the chicken reaches an internal temperature of 165°F before serving.