Prepare the Olive Salad: In a large mixing bowl, combine the green olives, black olives, giardiniera, roasted red peppers, capers, celery, and minced garlic. Pour in the extra virgin olive oil and red wine vinegar. Add the dried oregano and red pepper flakes. Season with freshly ground black pepper to taste. Mix thoroughly and refrigerate for at least 1 hour, preferably overnight.
Assemble the Sandwich: Slice the round loaf of Italian sesame seed bread horizontally. Spread half of the prepared olive salad over the bottom half of the bread. Layer the meats and cheeses over the olive salad, starting with mortadella, followed by salami, capicola, provolone, and mozzarella. Spread the remaining olive salad over the layered fillings. Place the top half of the bread over the fillings and press down gently.
Press and Serve: Wrap the assembled sandwich tightly in plastic wrap and place it under a heavy skillet or weight. Let it press for at least 30 minutes. After pressing, unwrap the sandwich and cut it into wedges or slices. Serve at room temperature or slightly chilled.
