Cut up any large frozen vegetables. In a 3-quart saucepan combine frozen vegetables, onion, chicken or vegetable broth, garlic salt, and white pepper. Bring mixture to boiling; reduce heat. Cover and simmer for 5 to 7 minutes or until vegetables are crisp-tender.
Stir milk and half-and-half or light cream into the vegetable mixture. Heat just to boiling. Add the undrained oysters. If using fresh oysters, cook over medium heat about 5 minutes or until edges curl, stirring frequently. If using canned oysters, cook until heated through.
Spoon into soup bowls. Top each serving with some of the margarine or butter. Makes 4 to 6 servings.