Oregano leaf is very safe. The German Commission E and American Herbal Products Association both state there are no known risks with oregano leaf;¹² neither of these references mentions oregano oil.

Due to the lack of human research and the highly concentrated nature of oregano volatile oil, there is potential for harm from its use; therefore, until its internal use in humans has been proven safe, it should taken with caution if not recommended by a healthcare professional.¹³ Volatile oils are generally considered contraindicated in pregnancy as they likely reach the baby and may cause harm.¹⁴ Topically, the volatile oil of oregano may be moderately irritating to skin and can be a potent mucous membrane irritant. It should not be applied topically to mucous membranes in greater than a 1% concentration.¹⁵ Children less than two years of age and people with damaged or very sensitive skin should not use the oil topically.¹⁶