1. To prepare olive relish: Combine parsley, olives, celery, garlic, oregano, lemon juice, oil, salt and pepper in a small bowl.
2. To grill tuna: Preheat grill to medium-high.
3. Rub tuna all over with oil and season with salt and pepper. Grill the tuna until seared on both sides and just cooked through, about 4 minutes per side. Serve with Olive Relish and lemon wedges.