In a large pot, heat the vegetable oil over medium-high heat.
Slowly add the flour, whisking constantly, and keep cooking until it turns a dark brown.
This should take about 15-20 minutes.
Add the chopped onion, bell pepper, and celery to the roux.
Cook them down for about 5 minutes, stirring often.
Add the minced garlic and give it another minute of cooking.
Pour in the shrimp stock, then add thyme, bay leaves, Worcestershire sauce, salt, and cayenne pepper.
Bring the mixture to a boil, reduce to a simmer, and let it go for an hour.
Skim any foam or oil that rises.
Add the shrimp and catfish pieces to the pot and cook for an additional 2 minutes.
Stir often to keep the gumbo smooth and delicious.
Finally, add the oysters and keep stirring while they cook for another 5 minutes.
Taste before serving and adjust seasoning if needed.