Combine the oil and flour in a large cast-iron or enameled cast-iron Dutch oven over medium heat. Stirring slowly and constantly for 20 to 25 minutes, make a dark brown roux, the color of chocolate.
Add the onions, bell peppers, celery, salt, cayenne, and bay leaves. Cook, stirring occasionally, for about 10 minutes, or until very soft. Add the water and mix to blend with the roux. Add the crabs and simmer, uncovered, stirring occasionally, for 1 1/2 hours. If you are not using the crabs, simmer the roux mixture for the same length of time. Add the shrimp and crabmeat and cook for 15 minutes. Add the oysters, green onions, and parsley and cook for 2 to 3 minutes, or until the edges of the oysters curl.
Remove from the heat. Remove the bay leaves.
Serve with the filé powder passed at the table for guests to thicken the gumbo to their personal taste.
Yield: 6 servings