A study was carried out on the essential oil of Origanum vulgare ssp. hirtum (Link) Ietswaart taken from thirteen wild populations found in different parts of Sicily, Italy. The aim of the study was to evaluate the relationship that exists between essentials oil extracted from Sicilian wild populations of O. vulgare ssp. hirtum (Link) Ietswaart and various weather/climate and topographic factors in order to provide more information on aspects that influence the production potential of this species. The essential oils from the samples were extracted by hydrodistillation and analyzed by a combination of gas chromatographyâflame ionization detector (GCâFID) and GCâmass spectrometry (GCâMS); seventy-one components were fully identified, however, three main components were characteristic of the examined samples accounting for a significant amount of all oils: thymol (24.0â54.4%), Î³-terpinene (9.8â30.5%) and p-cymene (5.2â18.7%). Analyses highlighted a different qualitative and quantitative essential oil composition compared with other wild populations in the Mediterranean area and in relation to the different weather/climate and topographic characteristics of the collection sites. These differences are probably due to a mixture of genetic and environmental factors, which influence secondary metabolite biosynthesis: the basis of essential oils.