Place chicken in a large Dutch oven. Cover with 14 to 16 cups water; add 2 bay leaves, and bring to a boil over medium-high. Reduce heat to medium-low, and simmer until chicken is tender, 1 to 1 1/2 hours. Transfer chicken to a bowl. Pour stock through a fine wire-mesh strainer, and discard solids. Cool stock 30 minutes, and skim fat. Reserve 8 cups of skimmed stock. Remove and discard chicken skin and bones. Chop chicken into large bite-size pieces.

Cook sausage in a Dutch oven over medium until browned, about 8 minutes; drain on paper towels. Add vegetable oil to Dutch oven, and heat over medium-high. Gradually add flour, whisking con­stantly. Cook, whisking constantly and reducing heat as necessary to keep roux from burning, until a dark mahogany color, 25 to 35 minutes.

Add onions, bell peppers, and celery to roux, and cook, stirring constantly with a large wooden spoon, until vegetables begin to soften, 8 to 10 minutes. Stir in chicken and sausage. Gradually add 8 cups reserved chicken stock, and stir well. Add thyme, Creole seasoning, salt, pepper, oregano, sage, Worcestershire sauce, and remaining 4 bay leaves; stir well. Bring to a boil, and boil 5 minutes. Reduce heat to medium-low, and simmer, uncovered, 1 to 3 hours. (If gumbo gets too thick, add a little more reserved chicken broth or water. If it is too thin, continue to simmer uncovered. Because there is pork sausage in this gumbo, skim off any excess fat that rises to the top.)

About 30 minutes before serving, add scallions and parsley. Cover and cook 15 minutes. Add oysters and shrimp; cook, uncovered, until oysters begin to curl, about 2 minutes.

Stir in desired amount of hot sauce. Remove gumbo from heat; let stand 10 minutes. (It will stay hot for a long time.) Add salt and pepper to taste. Serve over cooked white rice with filé powder and French bread.