Combine the oil and flour in a large cast iron or enameled cast iron Dutch oven over medium heat. Stirring slowly and constantly, make a dark brown roux, the color of chocolate, 20-25 minutes. Add the onions, bell peppers, celery, salt, cayenne, and bay leaves. Cook, stirring occasionally, until very soft, about 10 minutes. Add the water and stir to blend with the roux. Reduce the heat to medium-low and simmer for 1-½ hours.
Add the shrimp and crabmeat and cook for 15 minutes. Add the oysters, green onions, and parsley and cook until the edges of the oysters curl, 2-3 minutes. Remove the bay leaves.
Serve immediately over steamed long grain rice in soup bowls.
Yields 6 servings.