Oregano (Origanum vulgare L.) is one of the most commercially important herbs in the Lamiaceae family. It is a rich natural source of bioactive components including phenolic glucosides, flavonoids, tannins, sterols, triterpenes, resins and essential oil. In the present study, the variation of the essential oil compositions among seven populations of Iranian oregano, originating from different bioclimate and geographical zones was investigated over two harvest years (2014 and 2015), and under controlled soil and climate conditions. The essential oil content showed a wide variability, ranging from 0.12% to 1.76% (v/w), correlated to the chemical profile. GC-FID and GC–MS analyses of the essential oils characterized a total of forty-two constituents in oregano populations. Carvacrol (0.3–46.8%), linalyl acetate (0.2–44.3%), (Z)-α-bisabolene (0.0–40.3%), (E)-β-caryophyllene (0.0–24.0%), and caryophyllene oxide (0.1–21.3%) were identified as the main components of the essential oils, depending on the population and harvest year. The highest amounts of these components were recognized in the essential oil of Baneh, Rasht, Gilan, Kaleybar and Ardabil populations, respectively. According to cluster and principal component analyses (PCA), the studied populations were grouped into four main chemotypes: i.e., chemotype I (carvacrol), chemotype II ((Z)-α-bisabolene), chemotype III (linalyl acetate), chemotype IV (caryophyllene oxide/germacrene D/(E)-β-caryophyllene). Variability of essential oil constituents in oregano populations studied can primarily be explained by differences in efficiency and/or activity of the methylerythritol 4-phosphate (MEP) and mevalonate pathways. Intraspecific chemical variability in Iranian oregano provides possibility of selection of those batches with specific aromas and chemical profiles for industrial intentions. The results also provided new insight for development of effective conservation strategies, domestication and breeding programs in Ir
