Hmm, the free online dictionary says... Acronym Definition YOS Years Of Service YOS Oil Storage Barge (Non Self-Propelled) YOS Your Operating System YOS Your Obedient Servant (closing salutation chiefly 18th century British) YOS Yahoo Online Status (Yahoo Messenger) YOS Yttrium Ortho-Silicate
but I guess it could also be as the urban dictionary says...a derivative of the word "yas", used to express extreme joy, enthusiasm, or satisfaction.