In a 2-gallon stockpot, heat oil over medium-high heat. Once oil is hot, add flour and, using a wire whisk, stir constantly until roux is golden brown, being careful not to scorch. Should black specks appear, discard and begin again.
Add onions, celery, bell pepper, and garlic, and saute for 3 to 5 minutes, or until vegetables are wilted. Add duck and andouille sausage, blending into vegetable mixture. Add oyster liquid and chicken stock, 1 ladle at a time. Bring to a rolling boil, reduce to simmer and cook approximately 2 hours. When duck is tender, add oysters, and cook an additional 10 minutes. Add green onions and parsley. Season with salt, pepper, and pepper sauce. Serve over steamed white rice.
Recipe courtesy of Lafitte's Landing at Bittersweet Plantation, Louisiana