Stir together celery, olives, parsley, garlic, tomatoes, 4 tablespoons oil, vinegar, and pepper in a bowl.
Cut bread horizontally in half with a serrated knife. Remove soft centers of both halves to make room for filling, leaving 1/2-inch-thick shell. Drizzle both halves with remaining 2 tablespoons oil. Spread half of the olive mixture in each half of bread, gently pressing down to help adhere. Layer cheese, salami and ham on bottom half of bread. Cover with top half of bread. Wrap sandwich well in plastic wrap, then foil. Refrigerate for at least 4 hours.
Let sandwich stand at room temperature for 30 minutes before cutting into wedges.