1.Combine oil, lemon juice, garlic, oregano, and pepper to taste in a small bowl. Mix thoroughly.
2.Place chicken breasts between two sheets of wax paper and pound lightly with a mallet to flatten slightly. Transfer chicken to a dish. Add marinade and refrigerate 30 minutes.
3.Turn on broiler. Place breasts in a shallow baking dish. Brush with half the marinade. Broil 4-5 minutes. Turn, brush with remaining marinade, and broil another 4-5 minutes, or until browned.
4.Serve with pan juices. Garnish with lemon slices and sprinkle with parsley.