Heat oven to 450 degrees F.
Place onion, carrots and celery inside turkey cavity. Twist wing tips behind back. Tie up legs or tuck them into flap of skin. Place turkey, breast-side up, on rack in large roasting pan. Brush with oil. Roast at 450 degrees F. for 30 minutes.
Reduce oven temperature to 350 degrees F. Roast turkey at 350 degrees F., basting every 15 minutes with pan liquid, for 1-1/2 hours or until instant-read thermometer inserted into thickest part of thigh registers 180 degrees F. When turkey is 3/4 done, loosely cover breast and top of drumsticks with lightweight foil to prevent overcooking. Remove from oven; tent with foil. Let stand 20 minutes before carving. Makes 12 servings.