Oliver Wood: Etym: With a small o, an olive tree. The name is more likely from the Old German Alfihar, "elf-host".
Ollivander, Mr.: Proprietor of a wand shop in Diagon Alley. According to its sign, his family have been "Makers of Fine Wands since 382 BC".
Etym: OED thinks this is an error for olivaster, meaning "olive-colored" or "having an olive complexion".
One Thousand Magical Herbs and Fungi: A first-year textbook for Herbology.
Order of Merlin: An award given by the Ministry of Magic; the highest honor available to a British wizard.
There is a real-life Order of Merlin, but it's nothing like the fictional one. It's given by the International Brotherhood of Magicians for 25 years of membership.
owls: Popular as familiars, owls are also the basis of the wizard postal system, able to find anyone, anywhere, to deliver their mail.
Owlery: The place where the Hogwarts-owned owls, and sometimes student ones, roost when not delivering mail.