To make the dressing, in a small bowl combine the vinegar, garlic, olive oil, onion, celery and pepper. Stir to mix evenly. Cover and refrigerate until needed.
Prepare a hot fire in a charcoal grill or heat a gas grill or broiler. Away from the heat source, lightly coat the grill rack or broiler pan with cooking spray. Position the cooking rack 4 to 6 inches from the heat source.
Rub the chicken breasts with garlic, then discard the cloves. Grill or broil the chicken until browned and just cooked through, about 5 minutes each side. Transfer the chicken to a cutting board and let rest 5 minutes before slicing into strips.
Arrange 2 cups lettuce, 4 olives and 1/4 of the sliced oranges onto 4 plates. Top with 1/4 of the chicken strips and drizzle with dressing. Serve immediately.