Preheat the oven to 425°F. Lightly oil the pan with cooking oil.
Slice the chicken, bell peppers, and red onion into ¼-inch strips so they cook evenly. If the chicken breasts are especially thick, slice them in half before cutting into strips.
Add the chicken and vegetables to the sheet pan in an even layer.
In a small bowl, mix together olive oil, oregano, cumin, garlic powder, paprika, cayenne pepper, and salt.
Pour the mixture over the sliced chicken, peppers, and onions. Gently toss them with the oil and spice mixture.
Place the sheet pan in the oven for 25 to 30 minutes or until the chicken is cooked through. You can check the chicken with a meat thermometer to ensure it has reached 165°F in the center.
Serve with warm soft tortillas and enjoy! You can also squeeze additional fresh lime juice over top of the fajitas at the table if desired.