Oblansk (GoF ch. 8): Something close to the name of the Bulgarian Minister of Magic.
Etym: The closest thing I can find is Obolensk or Obolensky, the latter being a habitation name derived from the former.
Obalonsk (GoF ch. 8): Something else close to the name of the Bulgarian Minister of Magic.
Etym: See above.
Obliviate: The incantation for a Memory Charm.
Etym: Just the English word.
Obliviator: A member of the Accidental Magic Reversal Squad.
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.
Ogg (GoF ch. 31): The groundskeeper before Hagrid.
Etym: Anglicized form of a nickname from the Gaelic óg "young", used to distinguish the younger of two bearers of the same given name. Also Australian/New Zealand slang for "shilling".
Olde and Forgotten Bewitchments and Charmes: A book in the Hogwarts library.
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".
Olympe Maxime: Etym: Probably related to Olympus, in Greek legend, the highest mountain in the world.
Omnioculars: Magical binoculars that not only enhance vision but offer instant replay and advanced graphics.
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.
Orla Quirke: Etym: No info.
Ottery St. Catchpole: The fictional village where The Burrow is located.
Otto Bagman: Etym: From Old German auda "rich". The name of various Central European kings and emperors.
Ouagadougou: The capital of Burkina Faso, where Gilderoy Lockhart claims to have halted a series of supernatural attacks.
Owen Cauldwell: Etym: From Gaelic Eoghan "a youth". The name of several Welsh princes.
owls: Popular as familiars, owls are also the basis of the wizard postal system, able to find anyone, anywhere, to deliver their mail. In places where owls are not native, other birds may be used.
O.W.L.s: Ordinary Wizarding Levels.
Owlery: The place where the Hogwarts-owned owls, and sometimes student ones, roost when not delivering mail.
Owl Treats: What it sounds like.