If you're very careful, it can be safe to let your cat nibble on the occasional olive. But there are a few things you should keep in mind before feeding olives to your cat.
First, olives have a high fat content, and they often have a high sodium content as well. Foods with high sodium content—especially when eaten in large amounts over time—can cause an imbalance in your cat's electrolytes, leading to dehydration, or even more severe health problems such as kidney damage. And excessive fat consumption can lead to digestive problems like stomach upset and diarrhea, and even obesity.
Moreover, olives contain pits, which can cause choking or gastrointestinal damage. And olives also contain a compound called oleuropein, which can lead to digestive issues such as vomiting, diarrhea, and stomach upset in some felines.
All that being said, cats can still gnaw on the occasional olive so long as you do the following:
Feed them the unsalted variety
Carefully remove the pit and make sure no remnants remain
Feed them to your cat only in small amounts and not regularly
While some cats may show no adverse reactions after eating small amounts of black or green olives as an occasional treat, the safest thing you can do is avoid feeding them olives altogether. Truth be told, your cat won't be missing out on one of life's great pleasures by abstaining from olives. As obligate carnivores, cats need to get most of their nutrients from meat anyway.
Before introducing a new food item into your pet's diet, consult with your veterinarian first. If your cat eats olives (or any food) and is showing signs of illness such as lethargy, diarrhea, or vomiting, reach out to your veterinarian immediately. If they require veterinary care related to eating any unsafe foods, pet insurance can help cover the cost of treatment.