Preheat the grill. Season the lamb with olive oil, salt, pepper and oregano. Alternating with the lamb, potatoes and onions, place on each skewer. In a mixing bowl, combine the tomatoes, cucumbers, garlic, parsley and olives. Season with a drizzle of the extra virgin olive oil, salt and pepper. Mix well and set aside. Place the lamb on the grill and cook for 3 to 4 minutes on each side for medium rare. Remove from the grill and cool for a couple of minutes. To serve, place the kabobs on a large platter. Spoon the tomato mixture over the kabobs. Serve with the warm pita bread and a wedge of lemon.