Make stock from the shells and heads of peeled shrimp. Heat oil in skillet then add flour to make a roux. Stir constantly until very brown. Lower heat, add onions, peppers, thyme, basil and bay leaves. Slowly add the stock stirring constantly. Bring to a boil. Add sausage, crabs, shrimp and catfish. Lower heat to simmer, then add okra, garlic and season to taste. At the end add oysters. If gumbo is too thick add oyster water. If you need to, additional seasonings can be added to satisfy your personal taste. Serve over steamed rice.