Cut steak into bite-size pieces. Place steak bites into a resealable plastic bag.
Mix together oyster sauce, soy sauce, sweet soy sauce, sesame oil, and brown sugar in a bowl. Pour mixture over the steak pieces and mix to coat with marinade. Marinate for 30 minutes.
Preheat an air fryer to 400 degrees F (200 degrees C).
Line the air fryer basket with a perforated parchment round. Drain marinade from steak and set steak pieces onto the parchment.
Air-fry until steak bites are cooked to your desired doneness, about 6 minutes, shaking the basket at the 3-minute mark.
Remove steak bites from the air fryer and sprinkle with toasted sesame seeds before serving.