Whisk together vinegar, oregano and garlic in a large bowl until blended. Gradually whisk in oil. Stir in green olives, kalamata olives, roasted red bell peppers and black pepper.
Using a serrated knife, slice 1 inch from top of bread loaf. Split loaf horizontally, and scoop out soft bread from both halves, leaving a 1/2-inch-thick shell. (Reserve soft center of loaf for another use, if desired.).
Spread half of the vinaigrette in bottom half; layer with ham, salami, mortadella, cheese, onion and arugula. Spread remaining vinaigrette on top half of bread, and place vinaigrette side down on arugula. Cut the sandwich into 6 even wedges.