Place chicken in a resealable plastic bag set in shallow dish. For marinade, in small bowl combine lemon juice, oil, oregano, garlic, and pepper. Pour over chicken. Seal bag; turn to coat chicken. Marinate, refrigerated for 4 to 24 hours, turning bag occasionally.
Meanwhile, in medium bowl combine cucumbers, tomatoes, and onion; set aside.
Drain chicken, discarding marinade. Place chicken on rack of uncovered grill directly over medium coals. Grill 12 to 15 minutes or until chicken is tender and no longer pink (170 degrees F), turning once.
Transfer chicken to cutting board; cut into bite-size pieces. Toss with cucumber mixture. Serve on salad greens. Drizzle with salad dressing. Sprinkle feta cheese and olives. Makes 4 servings.