Trim any excess fat from the chicken thighs and place them in a mixing bowl.
Add the olive oil, minced garlic, dried oregano, lemon zest, lemon juice, salt, black pepper, and onion powder to the bowl and stir until the chicken is evenly coated.
Cover the bowl with plastic wrap and refrigerate to marinate for at least 20 minutes or up to 2 hours.
Preheat the air fryer to 380°F (if your model requires preheating).
Arrange the marinated chicken in a single layer in the air fryer basket without overlapping.
Cook at 380°F for 15–18 minutes, flipping halfway through, until the chicken is golden and reaches 165°F internally.
Remove the chicken, garnish with chopped fresh parsley and optional crumbled feta, and serve immediately.