The primary difference between Greek oregano and common oregano lies in their precise botanical classification. Both belong to the Lamiaceae (mint) family and the genus *Origanum*, but they are distinct species. True Greek oregano is scientifically classified as *Origanum vulgare* subsp. *hirtum*. It is often also referred to as *Origanum heracleoticum*. In contrast, the plant most frequently labeled as "common oregano" in gardens and stores is *Origanum vulgare*. This distinction is crucial, as *vulgare* translates to "common," making this species the baseline or wild type of oregano, which is generally considered to be of inferior culinary quality compared to its Greek cousin.
The two plants can be distinguished by several physical characteristics. Greek oregano (*O. vulgare* subsp. *hirtum*) is a hardy perennial herb with dense, hairy (pubescent), and often slightly grayish-green leaves. The stems are also notably hairy. Its flowers are white and highly attractive to pollinators. Common oregano (*O. vulgare*), on the other hand, typically has broader, brighter green leaves that are less hairy or even smooth (glabrous). The flowers of common oregano are most frequently pinkish-purple, though color can vary. The overall growth habit of Greek oregano is also usually more compact and low-growing compared to the common variety, which can become somewhat leggy.
The most significant difference for culinary use is found in their chemical composition, specifically the concentration of essential oils. The potency and flavor of oregano are dictated by compounds like carvacrol and thymol. Greek oregano is prized because it has a very high concentration of carvacrol, often exceeding 80%. This results in its signature robust, pungent, and peppery flavor with a slight bitterness, which holds up extremely well to cooking. Common oregano (*O. vulgare*) has a much more variable and generally lower concentration of these essential oils. Its flavor is consequently milder, sweeter, and less intense. It can sometimes be almost flavorless, leading to its reputation as a less desirable culinary herb.
While both plants are perennial herbs adapted to Mediterranean climates, their preferred conditions reflect their origins. Greek oregano subspecies *hirtum* thrives in poor, dry, rocky, well-drained soils under full sun. These stressful conditions are believed to contribute to the development of its high concentration of potent essential oils. Common oregano (*O. vulgare*) is more adaptable and can tolerate a wider range of conditions, including slightly more moisture and less-than-perfect drainage. This hardiness and adaptability are why it is so commonly found naturalized in gardens outside of its native habitat, but this often comes at the expense of flavor intensity.
