Pour oil into a large skillet. Place over low heat.
When oil is hot and shimmering, add celery, onions, peppers, okra, oregano, thyme, and bay leaves. Cook, stirring occasionally, until vegetables are very soft, about 45 minutes.
While vegetables are cooking, pour stock in a large heavy pot. Place over low heat and warm.
When vegetables are done, transfer to warmed stock.
Add Ro-Tel, diced tomatoes, tomato paste, crab boil and sugar.
Season to taste with salt and pepper.
Increase heat to medium, cover, and simmer for 30 minutes.
Add shrimp, crab and oysters. Simmer, stirring occasionally for 20 - 30 minutes.
Whisk filé powder into water. Pour mixture into gumbo.
Stir in reserved roux. Continue to simmer for 5 minutes. Remove from heat.
Discard bay leaves.
Serve gumbo by itself or over rice, as you like.