Oregano is a perennial herb, growing from 20–80 cm (7.9–31.5 in) tall, with opposite leaves 1–4 cm (0.39–1.57 in) long. The flowers are purple, 3–4 mm (0.12–0.16 in) long, produced in erect spikes. It is sometimes called wild marjoram, and its close relative, O. majorana, is known as sweet marjoram.
The word oregano is derived from Greek ὀρίγανον (orī́ganon), from which the Spanish orégano, and Latin orīganum.  This is a compound Greek term that consists of όρος (óros) meaning "mountain", and γάνος (gános) meaning "brightness", thus, "brightness of the mountain". 
Oregano is related to the herb marjoram, sometimes being referred to as wild marjoram. Oregano has purple flowers and spade-shaped, olive-green leaves. It is a perennial,   although it is grown as an annual in colder climates, as it often does not survive the winter.   Oregano is planted in early spring, the plants being spaced 30 cm (12 in) apart in fairly dry soil, with full sun. Oregano will grow in a pH range between 6.0 (mildly acidic) and 9.0 (strongly alkaline), with a preferred range between 6.0 and 8.0. It prefers a hot, relatively dry climate, but does well in other environments. 
Many subspecies and strains of oregano have been developed by humans over centuries for their unique flavours or other characteristics. Tastes range from spicy or astringent to more complicated and sweet. Simple oregano sold in garden stores as Origanum vulgare may have a bland taste and larger, less-dense leaves, and is not considered the best for culinary use, with a taste less remarkable and pungent. It can pollinate other more sophisticated strains, but the offspring are rarely better in quality.
The related species, Origanum onites (Greece, Turkey) and O. syriacum (West Asia), have similar flavours. A closely related plant is marjoram from Turkey, which differs significantly in taste though, because phenolic compounds are missing from its essential oil. Some varieties show a flavour intermediate between oregano and marjoram.
Accepted subspecies: 
- O. v. subsp. glandulosum (Desf.) Ietsw. - Tunisia, Algeria
- O. v. subsp. gracile (K.Koch) Ietsw. (= O. tyttanthum) has glossy green leaves and pink flowers. It grows well in pots or containers, and is more often grown for added ornamental value than other oregano. The flavor is pungent and spicy.  - Central Asia, Iran, India, Turkey, Afghanistan, Pakistan.
- O. v. subsp. hirtum (Link) Ietsw. - (Italian oregano, Greek oregano) is a common source of cultivars with a different aroma  from those of O. v. gracile. Growth is vigorous and very hardy, with darker green, slightly hairy foliage. Generally, it is considered the best all-purpose culinary subspecies. - Greece, Balkans, Turkey, Cyprus
- O. v. subsp. virens (Hoffmanns. & Link) Ietsw. - Morocco, Spain, Portugal, Balearic Islands, Canary Islands, Azores, Madeira
- O. v. subsp. viridulum (Martrin-Donos) Nyman - widespread from Corsica to Nepal
- O. v. subsp. vulgare - widespread across Europe + Asia from Ireland to China; naturalized in North America + Venezuela
Example cultivars of oregano include:
- 'Aureum' – Golden foliage (greener if grown in shade), mild taste: It has gained the Royal Horticultural Society's Award of Garden Merit. 
- 'Greek Kaliteri' – O. v. subsp. hirtum strains/ landraces, small, hardy, dark, compact, thick, silvery-haired leaves, usually with purple undersides, excellent reputation for flavor and pungency, as well as medicinal uses, strong, archetypal oregano flavor ( Greek kaliteri: the best)
- 'Hot & Spicy' – O. v. subsp. hirtum strain
- 'Nana' – dwarf cultivar
Cultivars traded as Italian, Sicilian, etc. are usually hardy sweet marjoram ( O. ×majoricum), a hybrid between the southern Adriatic O. v. subsp. hirtum and sweet marjoram (O. majorana). They have a reputation for sweet and spicy tones, with little bitterness, and are prized for their flavor and compatibility with various recipes and sauces.
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Oregano is a culinary herb, used for the flavor of its leaves, which can be more flavorful when dried than fresh. It has an aromatic, warm, and slightly bitter taste, which can vary in intensity. Good-quality oregano may be strong enough almost to numb the tongue, but cultivars adapted to colder climates may have a lesser flavor. Factors such as climate, season, and soil composition may affect the aromatic oils present, and this effect may be greater than the differences between the various species of plants. Among the chemical compounds contributing to the flavour are carvacrol, thymol, limonene, pinene, ocimene, and caryophyllene. 
Oregano's most prominent modern use is as the staple herb of Italian cuisine. Its popularity in the U.S. began when soldiers returning from World War II brought back with them a taste for the "pizza herb", which had probably been eaten in southern Italy for centuries.  There, it is most frequently used with roasted, fried, or grilled vegetables, meat, and fish. Oregano combines well with spicy foods popular in southern Italy. It is less commonly used in the north of the country, as marjoram generally is preferred.
In Turkish cuisine, oregano is mostly used for flavoring meat, especially for mutton and lamb. In barbecue and kebab restaurants,[ clarification needed] it can be usually found as a condiment, together with paprika, salt, and pepper.
Oregano oil has been used in folk medicine over centuries.   Oregano essential oil is extracted from the leaves of the oregano plant. Although oregano or its oil may be used as a dietary supplement, there is no clinical evidence to indicate that either has any effect on human health.  
In 2014, the US Food and Drug Administration (FDA) warned a Utah company, Young Living, that its herbal products, including oregano essential oil, were being promoted to have numerous unproven anti-disease effects, and so were being sold as unauthorized misbranded drugs subject to seizure and federal penalties.  Similar FDA warning letters for false advertising and unproven health claims about oregano essential oil products were published in 2017 and 2018.  
The essential oil of oregano is composed primarily of monoterpenoids and monoterpenes, with the relative concentration of each compound varying widely across geographic origin and other factors. Over 60 different compounds have been identified, with the primary ones being carvacrol and thymol ranging to over 80%, while lesser abundant compounds include p-cymene, γ-terpinene, caryophyllene, spathulenol, germacrene-D, β-fenchyl alcohol and δ-terpineol. 
Drying of the plant material affects both quantity and distribution of volatile compounds, with methods using higher heat and longer drying times having greater negative impact. A sample of fresh whole plant material found to contain 33 g/kg dry weight (3.1 g/kg wet) decreased to below a third after warm-air convection drying. Much higher concentrations of volatile compounds are achieved towards the end of the growing season. 
- Cuban oregano or oregano poleo (Plectranthus amboinicus, formerly Coleus aromaticus), is also of the mint family (Lamiaceae). Sometimes also called "Mexican mint" or "Mexican thyme", it has large and somewhat succulent leaves. Not just a Latin American plant, it is also grown and used throughout the tropics, including Africa and Southeast Asia.
- Mexican oregano (Lippia graveolens) is not in the mint family, but in the closely related vervain family (Verbenaceae), that includes e.g. the lemon verbena (Aloysia citrodora). It is a highly studied herb[ citation needed] that is said[ by whom?] to be of some medical use and is common in curandera (female shamanic practices) in Mexico and the Southwestern United States. The flavor of Mexican oregano has a stronger savory component instead of the piney hint of rosemary flavor in oregano, and its citrus accent might be more aromatic than in oregano. It is becoming more commonly sold outside of Mexico, especially in the United States, where it is an important source of dried oregano. It is sometimes used as a substitute for epazote leaves; this substitution would not work the other way round. Epazote has a lighter, and even more savory flavor and citrus accent than Mexican oregano, and a very strong sweet tarragon-like flavor which might be its strongest component. When Mexican oregano is substituted for epazote, the base flavor less the tarragon-like sweetness is provided, and other corrections to the recipe may cover for the missing tarragon component of epazote, or the dish may be served without that flavor component. If epazote were substituted for Mexican oregano, many dishes would be overpowered by the tarragon accent.
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