Whisk the vinegar and oregano in a large bowl to blend. Gradually whisk in the oil. Stir in the olives and roasted peppers. Season the vinaigrette to taste with salt and pepper.
Using a serrated knife, slice off the top 1 inch of the bread loaf. Hollow out the bottom and top halves of the bread. Spread some of the vinaigrette over the bread bottom. Layer the pot roast, cheese and prosciutto in the bottom half. Top with the onions, then the arugula. Spread the remaining vinaigrette over the bread top. Replace the bread top. (The sandwich can be made 1 day ahead. Wrap with plastic and refrigerate.)
Cut the sandwich into wedges and serve.