Oregano oil contains high levels of important compounds in its leaves and smaller stems. Oil producers dry the leaves and stems and then steam distill the plant material to extract as many compounds as possible.

Oregano oil contains:

carvacrol, the main active compound in oregano oil, and a type of antioxidant called phenol
thymol, which can help protect against toxins and fight fungal infections
While oregano oil contains a significant amount, the compound thymol occurs mostly in thyme. https://www.medicalnewstoday.com/articles/324203#what-is-oregano-oil