In a medium bowl, combine chopped green and black olives, capers, minced garlic, chopped parsley, and diced celery. Drizzle with olive oil and red wine vinegar, then sprinkle with oregano, black pepper, and crushed red pepper flakes. Mix thoroughly and let marinate for at least 30 minutes, preferably overnight in the fridge.

Cut your round Italian bread in half horizontally, creating a top and bottom layer. Consider scooping out some soft interior to prevent sogginess.

Place the bottom half of the bread on a large cutting board. Spoon half of the olive salad evenly over the bread. Layer slices of mortadella, capicola, and Genoa salami on top, followed by provolone and mozzarella cheese.

Spoon the remaining olive salad over the stacked meats and cheeses. Carefully place the top half of the bread on the sandwich, gently pressing down to compact the layers.

Wrap the sandwich tightly in plastic wrap. Weigh down with a heavy object and refrigerate for 1-2 hours, or ideally overnight.

Preheat your oven to 350°F (175°C). Wrap the sandwich in aluminum foil and bake for about 15 minutes to melt the cheeses and warm the meats.