Preheat oven to 350°. In a small bowl, combine the first five ingredients. Pat turkey dry; brush with oil. Sprinkle herb mixture over skin of turkey. Skewer turkey openings; tie drumsticks together.
Place flour in oven bag and shake to coat. Place the bag in a roasting pan; add celery and onion. Place turkey, breast side up, over vegetables. Cut six 1/2-in. slits in top of bag; close bag with tie provided.
Bake 2 to 2-1/2 hours or until a thermometer reads 180°. Remove turkey to a serving platter and keep warm. Let stand 15 minutes before carving. If desired, thicken pan drippings for gravy.