HEAT oven to 325°F.
REMOVE neck and giblets from turkey cavities. Rinse turkey; pat dry with paper towels. Free legs from tucked position. (Do not cut band of skin.) Using rubber spatula or hand, loosen skin over breast, starting at body cavity opening by legs. Spread 2 Tbsp. mustard inside body cavity; spread remaining mustard on meat under skin. Use wooden toothpicks to secure skin at opening. Return legs to tucked position; turn wings back to hold neck skin in place.
PLACE turkey, breast side up, on flat rack in shallow open pan; brush with oil.
BAKE 3 hours 30 min. to 3 hours 45 min. or until done (165°F), covering breast loosely with foil to prevent overbrowning if necessary. Let stand 15 to 20 min. before carving. Discard toothpicks before serving.