Heat oil in a 6–8 quart pot, then gradually whisk in the flour. Cook over medium heat for 20 minutes until very dark reddish brown. Stir constantly, especially towards the end. The best roux is quite dark, but don’t let it burn.
Stir the onion, celery, bell pepper, and garlic into the roux. Cook 10 minutes (or 15 minutes if using okra).
Slowly and gradually whisk in the shrimp stock.
Add the clam juice, oregano, thyme, bay leaves, salt, cayenne, pepper, and parsley. Simmer 15 minutes.
Add the Andouille sausage, and simmer for another 30–40 minutes.
Add the shrimp, oysters, and crawfish at the end, and simmer just until cooked through.