Although abundant chemical compounds have been isolated from oregano, the most important group from a commercial and application point of view, refers to its volatile oil. Oregano is the commercial name of those Origanum species that are rich in the phenolic monoterpenoids, mainly carvacrol and occasionally thymol (Azizi, 2010). EO is responsible for the most commercially important chemicals displaying its aroma, flavor, and pharmaceutical properties (Morshedloo et al., 2017). As presented in Table 2, a wide chemical diversity is found in the essential oil composition of O. vulgare in point of quantitative and qualitative, depending on geographical distribution or on the time of plant collection. In general, the plant is rich in monocyclic monoterpenes (carvacrol, thymol, ?-terpinene and p-cymene), sesquiterpenoids (?-caryophyllene, ?-bisabolene, spathulenol, germacrene-D) and acyclic monoterpenes (linalool, linalyl acetate, ?-myrcene and geraniol). Bicyclic monoterpenes (sabinyl compounds) have also been found in some species depending on the chemotype.