Preheat broiler to high. Place breast halves on flat surface and pound lightly to flatten slightly. Brush oil over bottom of baking dish large enough to hold breasts in one layer. Sprinkle chicken with pepper and arrange, skinned side up, in baking dish. Blend mustard and mayonnaise and spread equally over all breast halves. Place chicken about 12 inches below source of heat and broil for about 15 minutes, or until tops are golden brown and chicken is cooked through. If broiler pan cannot be placed a full 12 inches from heat source, place pan as far below as possible, reduce oven temperature to 375° and broil for 15 minutes.