In a 5-quart saucepan, heat oil over medium heat. Add flour and cook, stirring constantly, for about 30 minutes until roux turns dark brown and develops a nutty aroma. Add green peppers, celery, onion and garlic; cook for 10 minutes or until vegetables are tender. Add tomatoes with liquid, okra, water, chicken, ham, parsley, hot pepper sauce, salt and bay leaf. Over high heat, heat to boiling; reduce heat to medium. Simmer, uncovered, for 45 minutes, stirring occasionally. Add oysters, simmer 10 minutes longer. Discard bay leaf. Serve with cooked rice and additional hot pepper sauce, if desired.