For stuffing, slice sausage in half length-wise and grill on both sides. Reserve 2 halves for garnish and slice remaining sausage. Set aside.
Sauté celery and onions in butter until tender. Set aside to cool
Whisk eggs with 3½ cups of stock. Add sliced sausage, onions, celery, sage, marjoram, 1½ tablespoons Old Bay, salt, pepper, and bread. Mix well. Add more chicken stock if needed (should be very wet).
Spoon into large non-stick muffin pan. Cover loosely with foil and bake for 45 minutes. Remove foil and bake an additional 15 minutes, until tops are browned. Remove from oven, cool slightly, and remove from pan. Cool for reheating later or keep warm in a low oven.
For fried oyster topping, heat oil in fryer or large stock pot to 355 degrees.
Pull oysters from liquor and place in a bowl. Add buttermilk and remaining teaspoon of Old Bay.
In a separate bowl, mix flour and cornmeal, stirring well. Dredge oysters into flour mixture and shake off excess. Place oysters into oil and fry 1½ to 2 minutes.
Reheat muffins if not kept warm. Reheat reserved andouille sausage and finely slice. Assemble 2 to 3 fried oysters on each muffin, top with gravy if desired, and sprinkle with andouille.