Heat the oil in a large pot, add the flour, and cook the roux, stirring constantly, until it is a deep reddish-brown. Add the onion, bell pepper, celery and garlic. Cook, stirring constantly, until vegetables are tender. Add stock, seasonings and parsley. Bring to a boil, then lower heat and simmer for 30 minutes over low heat. Add the shrimp, oysteres and oyster liquor. Cook for 5 more minutes, just until the shrimp turn pink. Place 1/2 cup cooked rice in a shallow bowl, and ladle gumbo over and around it. Sprinkle filé on each serving to taste.