Olives are not toxic to cats, but have little nutritional value. Green olives and pimentos contain chemicals similar to those found in catnip, so some cats may be especially excited by olives.
Green olives and pimentos contain isoprenoids, which are similar to nepetalactone, which is found in catnip. Black olives do not contain the same compounds. As with the effects of catnip, some cats may become excited when given green olives or pimentos, while others are not at all affected. Adding olives to the diet may have minimal nutritional benefit, because olives contain unsaturated fats, but can cause diarrhea in some cats.