Melt butter in a large skillet over medium heat. Add onion and celery; cook 3 to 5 minutes or until tender. Transfer to a very large bowl; stir in bread cubes, salt, pepper and sage; set aside.
Drain oysters through a large strainer; transfer oysters to a bowl. Strain oyster liquid through a fine sieve into a separate bowl (you should have about 1/3 cup); set aside. Rinse oysters under cold running water to remove any sand or shells and coarsely chop. Add oysters to bowl with bread cube mixture.
Combine oyster liquid, milk and egg. Drizzle liquid mixture over the bread mixture to moisten, tossing lightly. (If necessary, add 1 or 2 tablespoons additional milk to moisten.) Use to stuff one 10- to 12-pound turkey. OR place stuffing in a 3-quart casserole. Cover and bake in a 325 degree F oven for 40 to 45 minutes or until an instant-read thermometer inserted in center reaches 165 degrees F. Makes 8 cups (8 to 10 servings).