Pat dry chicken with a paper towel. Massage oyster sauce then season both sides with garlic powder, onion powder and salt, and pepper on both sides. Marinate for 10 mins.
Brush the air-fryer basket with oil. Preheat to 200c/ 400f for 5 minutes.
Meanwhile, dredge each chicken with cornstarch. Coat well then shake off excess flour. Brush or spray chicken with coating on all sides. This will prevent floury skin.
Place the chicken in the basket in a single layer without overlapping, SKIN SIDE UP. You may need to work in batches if needed. Do not overcrowd.
Air fry for 18 minutes until skin is brown and crispy. Flip to the other side and air fry for 7 minutes or until crispy and cooked through. Note that cooking times may vary depending on the size of your air fryer.