Drain the oysters, reserving the juice, and set oysters aside. Place flour in a small bowl. Gradually add oyster juice, stirring with a wire whisk until blended; set aside.
Melt margarine in a Dutch oven over medium heat. Add onion, celery, and carrot, and sauté 5 minutes. Add milk and diced potato; bring to a simmer. Cover and cook 10 minutes. Add corn; cover and cook 5 minutes. Add oysters, oyster juice mixture, salt, hot sauce, and pepper; cook, uncovered, 6 minutes or until edges of oysters curl. Ladle into soup bowls, and top with green onions.