I've used the word "mossy" a thousand times to describe a fragrance, and certainly know the note well enough by this point in my addicition. But I'll admit it, I really have no idea what oakmoss actually is! I mean, I have an inkling, but I wouldn't bet money on it...
What I think it is: I think it actually IS a kind of moss...that grows on Oak trees?
What Wikipedia says: (drumroll please) Oakmoss (Evernia Prunasti) is a lichen (a combination of algae and fungus that work together to create a new distinct organism) that grows on the trunk and branches of oaks and other deciduous trees, and is usually harvested in South-Central Europe. Additionally, oakmoss that grows on pines has a distinctive "turpentine" odor. Itsmynature.net also adds that Oakmoss is most commonly used as a basenote in perfumery and is valued as a fixative, prolonging the life of other notes used in a composition by equalizing the evaporation rates of ingredients.
But what does it smell like? Most commonly used in fougeres and chypres, oakmoss is an earthy, deep aroma, sometimes slightly bitter, sometimes musty, with an organic woody undertone (imagine: mushrooms meet a wet tree branch).
Some of my favorite fragrances that use Oakmoss include: