Oysters themselves are not toxic to dogs. However, feeding oysters to a dog is inadvisable because of the possibility that the oysters consumed something that is toxic to canines, according to the Daily Puppy.
Dinoflagellates are a type of algae that is toxic to both humans and dogs. These hazardous algae can collect in the tissues of oysters while they filter feed. If enough dinoflagellates are eaten, they can cause paralytic shellfish poisoning in both humans and canines.
Other foods that owners should not feed dogs include avocados, onions, garlic and caffeine-containing beverages. Avocados contain persin, which is toxic to dogs. Large doses of onions and garlic can cause anemia in dogs, and caffeine is fatal in large amounts.