Preheat oven to 350 degrees. Shake flour in oven bag; place in roasting pan at least 2 inches deep. Add vegetables to oven bag. Remove neck and giblets from turkey. Rinse turkey; pat dry. Lightly stuff with your favorite stuffing recipe, if desired. Brush turkey with oil. Place turkey in oven bag on top of vegetables.
Close oven bag with nylon tie; cut six 1/2-inch slits in top. Insert meat thermometer through slit in bag into thickest part of inner thigh, not touching bone.
Bake 2 to 2 1/2 hours for a 12- to 16-pound turkey; 2 1/2 to 3 hours for a 16- to 20-pound turkey; 3 to 3 1/2 hours for a 20- to 24-pound turkey. Let stand in bag 15 minutes. If turkey sticks to oven bag, gently loosen bag from turkey before opening oven bag. Makes 12 to 32 servings.