This might uhm, sound stupid, but....

Are you sure it's leaking and it isn't just some oil that was stuck on the top of the oil cap when you opened it, and when you closed it, the oil that was already outside of the engine just ran down the cap?

Every time you open it, you should wipe your oil cap again, both inside and out, as well as the threads on the head cover. Then replace the cap. Otherwise, you'll get some slight drip from the side.