What is the difference between Mexican oregano and Italian oregano?

Neil Russo is correct in noting that Mexican Oregano is not the same plant as Mediterranean Oregano. But that's only the start of the story with Oregano.

Botanists and food scientists now believe that rather than any plant or plants, Oregano should be considered a suite of flavors, from many different plants that all bear some similarity to each other.

Origanum, a member of the Mint family, is the genus in which Italian Oregano (O vulgare) is found, but there are other species of Origanum as well as subspecies that each have slightly different flavors. Turkish and Greek Oregano, for instance, is O vulgare ssp hirtum.

Mexican Oregano is Lippia graveolens, a member of the Vervain family. Cuban Oregano is Plectranthus amboinicus, also a member of the Mint family, but with different aromatic oils than Oregano.

To keep things less than clear, Marjoram (Origanum majorana), a culinary herb that shares many of the same flavors as "Oregano" and Origanum vulgare is often known as "wild marjoram".