Bitter principle of olives. Nutriceutical with antioxidant props. Oleuropein is a chemical compound found in olive leaf from the olive tree (and leaves of privet) together with other closely related compounds such as 10-hydroxyoleuropein, ligstroside, and 10-hydroxyligstroside. All these compounds are tyrosol esters of elenolic acid that are further hydroxylated and glycosylated. Oleuropein and its metabolite hydroxytyrosol have powerful antioxidant activity both in vivo and in vitro and give extra-virgin olive oil its bitter, pungent taste. Oleuropein preparations have been claimed to strengthen the immune system.