In a medium bowl, combine the chicken, onion, bell pepper, garlic powder, chili powder, cumin, salt and pepper. Heat the oil in a large skillet over medium heat. Add the chicken mixture to the skillet and cook until the vegetables are tender.
Divide the mixture evenly among 4 tortillas. Sprinkle 1/2 cup of cheese on each quesadilla. Top with remaining 4 tortillas.
Heat a large skillet over medium heat. Place one quesadilla into the skillet and cook for 2-3 minutes per side or until golden brown and cheese is melted.