Pat the chicken cubes dry with paper towels and season with salt and pepper.
Place the chicken cubes in a large bowl with 1/2 cup cornstarch and toss until evenly coated.
Preheat the air fryer to 400°F (200°C).
Lightly spray the air fryer basket with oil and arrange the chicken in a single layer, ensuring pieces do not touch.
Air fry the chicken for 12 to 15 minutes, shaking the basket halfway through, until golden brown and internal temperature reaches 165°F.
While chicken is cooking, combine orange juice, sugar, soy sauce, rice vinegar, ginger, garlic, and chili flakes in a small saucepan over medium heat.
Bring the sauce to a simmer, then whisk in the cornstarch slurry and continue to simmer for 1-2 minutes until the sauce thickens and becomes glossy.
Transfer the cooked chicken to a large mixing bowl and pour the thickened orange sauce over the pieces.
Toss until the chicken is thoroughly coated.
Serve immediately, garnished with sliced green onions and toasted sesame seeds.