To prepare olive relish: Combine parsley, olives, celery, garlic, oregano, lemon juice, oil, salt and pepper in a small bowl.
To grill tuna: Preheat grill to medium-high.
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.
Make Ahead Tip: The olive relish (Step 1) will keep for up to 1 hour.
4 1/2 lean meat