Stir the green olives, black olives, giardiniera, vinegar, olive oil and oregano together and let the olive salad stand for 10 minutes.
Slice the loaf horizontally and pull only a thin layer of soft crumb from the top half so the filling has room.
Spread half the olive salad over the bottom, carrying the oily dressing all the way to the edge.
Layer the mortadella, salami, capicola, provolone and mozzarella evenly, folding the slices instead of stacking them into one dense block.
Spread the remaining olive salad under the top half and close the loaf.
Wrap the sandwich tightly in parchment, set a skillet on top and press it for 30 minutes at room temperature.
Unwrap the loaf and cut it into 6 wedges, keeping the layered filling inside each piece.