Set the air fryer to 350°F (175°C) and preheat for 3–5 minutes.
In a mixing bowl, whisk together the eggs, milk, Parmesan, garlic powder, oregano, salt, and pepper until fully combined and slightly frothy.
Stir in the cherry tomatoes, spinach, onion, and bell peppers.
Lightly spray a small, air-fryer-safe pan or dish with non-stick cooking spray.
Pour the egg mixture into the prepared pan. Place it in the air fryer basket and cook for 15–18 minutes.
Sprinkle the shredded mozzarella on top during the last 2 minutes of cooking.
Insert a toothpick into the center — if it comes out clean, the frittata is done.
Let it cool for 2–3 minutes before slicing. Serve warm and enjoy.