In a large, heavy skillet, make a roux by heating flour and oil over a medium flame. Mix thoroughly and stir constantly. Mixture should bubble slowly. Let cook a few minutes until light brown. Add shallots and celery.
Cook on a medium flame for about 10 minutes or until celery is soft. Add mushrooms, oysters, oyster liquor, and garlic. Cook slowly, uncovered, until mushrooms become tender and sauce thickens, about 15 to 20 minutes. Add parsley, pepper, and salt. Stir well. Serve hot with crackers, chips, or raw vegetables. Yields 3 cups.