Add the chili powder, smoked paprika, cumin, garlic powder, onion powder, oregano, salt, and black pepper to a small bowl. Stir until evenly combined.
Place the sliced chicken, red bell pepper, yellow bell pepper, and onion in a large bowl.
Pour the olive oil over the chicken and vegetables. Toss well to coat everything.
Sprinkle half of the spice mixture over the ingredients and toss thoroughly. Add the remaining seasoning and toss again until the chicken and vegetables are evenly coated.
Transfer everything to the air fryer basket, spreading the mixture out as evenly as possible.
Cook at 400°F (200°C) for 15 minutes. No preheating is necessary.
After 10 minutes, carefully toss or turn the chicken and vegetables with tongs.
After 15 minutes, check a thick piece of chicken to make sure it is fully cooked. If necessary, continue cooking for another 1–2 minutes.
Serve the air fryer chicken fajitas immediately with warmed tortillas, lime wedges, sour cream, guacamole, or your favorite toppings.