1. Using a sharp knife, cut a pocket in each chicken breast by cutting horizontally through the thickest portion to, but not through, the opposite side. Set aside.
2. In a small bowl, combine feta, artichoke hearts, roasted peppers, green onion, and oregano. Spoon evenly into pockets in chicken breasts. If necessary, secure openings with wooden toothpicks. Sprinkle chicken with black pepper.
3. Coat an unheated large nonstick skillet with cooking spray. Preheat skillet over medium heat. Add chicken. Cook for 12 to 14 minutes or until no longer pink (170 degrees F), turning once.