Heat broiler. With a rolling pin, flatten both halves of the buns. Place on a baking sheet and brush both sides with 2 tablespoons of the oil. Broil until golden brown, about 1 minute per side. In a bowl, combine the tomatoes, olives, oregano or basil, salt, pepper, and the remaining oil. Transfer the buns to a platter and spoon the tomatoes over the top.