Olives are not particularly toxic to dogs. Fresh olives are a better choice than canned, because canned olives have a lot of salt.
Dogs can eat olives, including fresh, canned and olive oil. However, olives are high in fat and canned olives also have a lot of salt. It is not good for dogs to have too much of either. Too much fat can cause pancreatitis, and too much salt can lead to high blood pressure and dehydration.
It is best if dogs only have a few fresh olives every now and then. Olive oil is considered a good fat for humans and dogs. A moderate amount of about a tablespoon a day mixed with food can help improve a dog's skin, fur and reduce inflammation.