Oregano is incredibly potent. Topically, it can cause sensitivity and should only be used in extremely low dilutions. It should also be mixed with other oils before diffusing, as it is extremely potent. Mix with a blend before diffusing, mix into a blend for topical application, or apply in max 2% dilution.
There is an interesting side note to Oregano: two things survived the nuclear bomb tests of the 1950s – cockroaches and yeasts. Nonetheless, researchers at Georgetown University proved that oil of wild oregano destroyed yeasts in petri-dishes.