Make a roux by combining the oil and flour in a deep pot and cooking over medium low heat until dark brown in color, about 12 minutes. Add the onions and mix well. Then, add the celery, parsley, sweet pepper, and green onions and mix well and cook for about 25 minutes.
Add the shrimp, oysters, oyster liquid, and crabmeat and mix together well. Add water, and cook for 1 hour. Add salt and pepper, to taste. Serve over rice.