Slice sausage into 3/4 inch slices and brown well in large skillet or brazier allowing fat to render out. Remove with slotted spoon and keep warm. In same skillet, make roux by adding oil and flour to rendered fat. Cook over low heat until very dark, almost to burning point, stirring constantly. This will take approximately 40 minutes; do not rush this process as a carefully made roux is essential to a fine gumbo.
When roux has reached desired color and has a rich, nut-like smell, add onions, celery, bell pepper and garlic. Let the vegetables cook in the roux until they have reached a crisp tender texture, approximately 15 minutes. Begin adding warm stock, one cup at a time, and stirring well until consistency is that of very thick soup.
At this time, you can combine roux/vegetable mixture, remaining stock, duck meat, reserved sausage and okra in a large stock pot. The oysters will not be added until gumbo is ready for service. Add file powder, creole seasoning, salt and pepper. Allow gumbo to simmer gently at least one hour, stirring occasionally and adjusting seasonings if necessary just before service. Cook rice in 3 cups salted water for approximately 20 minutes just before service.
Add the oysters to simmering gumbo and cook for no more than 3 minutes, ladle gumbo into soup plate, top with scoop of rice and sprinkle green onions over all.