Shake flour in Oven Bag; place in roasting pan at least 2 inches deep. Spray inside of oven bag with nonstick spray to reduce sticking, if desired.
Remove neck and giblets from turkey. Rinse turkey; pat dry. place onions and celery inside and outside of turkey. Brush turkey with oil and sprinkle with garlic salt and paprika. Place turkey in oven bag on top of vegetables.
Close oven bag with nylon tie; cut six 1/2-inch slits in top.
Bake 2 to 2 1/2 hours for a 12 to 16 lb. turkey, 2 1/2 to 3 hours for a 16 to 20 lb. turkey, and 3 to 3 1/2 hours for a 20 to 24 lb. turkey or until meat thermometer reads 180°F. Add 1/2 hour for stuffed turkey. For easy slicing, let stand in oven bag 15 minutes. If turkey sticks to oven bag, gently loosen oven bag from turkey before opening oven bag.