Obliviate: The incantation for a Memory Charm.
Etym: Just the English word.
Ogden's Old Firewhisky: A favorite of Gilderoy Lockhart.
Etym: Place in Yorkshire, from Old English aac "oak" + denu "valley"; probably just picked for alliterative value.
Olive Hornby: Etym: Olea europaea or Olea sativa. ECN: "In the Roman martyrology there is a St. Oliva, a virgin of Anagni of unknown date, and also a St. Oliva, venerated in the place so named and the patroness of olive-trees, which looks suspiciously as though she started life as a tutelary goddess."
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 Minute Feasts -- It's Magic!: A cookbook in Molly Weasley's collection.
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.
Order of Suspension: Issued by the governors of Hogwarts to temporarily remove Dumbledore.
Ordinary Wizarding Levels: Exams to demonstrate basic competence in a subject at graduation, equivalent to "O-levels" (ordinary levels) in British schools.
Ottery St. Catchpole: The fictional village where The Burrow is located.
Ouagadougou: The capital of Burkina Faso, where Gilderoy Lockhart claims to have halted a series of supernatural attacks.
owls: Popular as familiars, owls are also the basis of the wizard postal system, able to find anyone, anywhere, to deliver their mail.
O.W.L.s: Ordinary Wizarding Levels.
Owlery: The place where the Hogwarts-owned owls, and sometimes student ones, roost when not delivering mail.