Oregano oil is well known in the Mediterranean world for its anti-fungal and anti-microbial activity. Its use goes back to classical times, when it was used to slow down food spoilage. There are many species of oregano- the herbs which have been most widely used medicinally are Origanum vulgare and Origanum dictamnus. The former is found in Greece and the Eastern Mediterranean, while O. dictamnus is native to Crete. Other related herbs are thyme and marjoram, which are sometimes labelled incorrectly as oregano. Carvacrol is the predominant phenol in oil of oregano, whilst thymol is the predominant phenol in thyme and marjoram.
The major active constituents of oregano oil are carvacrol and thymol , which are isomeric phenols. There are many other compounds, which contribute to the anti-fungal and anti-microbial activity of oregano oil. It appears that these compounds act synergistically. Both carvacrol and thymol possess activity against both fungi and bacteria, carvacrol exerts higher activity against fungi while thymol is more active against gram-negative bacteria. These compounds also act as free radical scavengers and anti-oxidants.