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.
Obliteration Charm (OotP ch. 20): A spell to make something vanish.
Obliviate: The incantation for a Memory Charm.
Etym: Just the English word.
Obliviator: A member of the Accidental Magic Reversal Squad, or someone who specializes wiping Muggles' minds of things they weren't supposed to have seen.
Occlumency (OotP ch. 24): The art of blocking Legilimency.
Etym: Latin occluudere "to shut up, close" + mens "mind, intellect".
Occlumens: A wizard or witch who is proficient in Occlumency.
Octavius Pepper: Etym: Modification of Octavian.
Odo (HBP ch. 22): A legendary wizard, the subject of a long, tragic ballad.
Etym: From Old German auda "rich".
Office for the Detection and Confiscation of Counterfeit Defensive Spells and Protective Objects (HBP ch. 5): A newly formed authority headed by Arthur Weasley.
Official Gobstones Club (OotP ch. 7): A Gobstones league run by the Ministry of Magic.
Ogden, Bob (HBP ch. 10): The one-time head of the Magical Law Enforcement Squad.
Etym: See below.
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.
Ogden, Tiberius (OotP ch. 15): An elder of the Wizengamot who resigned in protest at the creation of the post of High Inquisitor.
Etym: See above; also the name of a an early 20th century linguist.
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, now missing. 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.
Oppugno (HBP ch. 14): An incantation to direct conjured creatures to attack.
Etym: From Latin for "fight against, attack".
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 the Phoenix, The: Dumbledore's anti-Voldemort alliance. Members and status:
Casualties of the first war: Edgar Bones, Caradoc Dearborn, Benjy
Fenwick, Alice Longbottom, Frank Longbottom, Marlene
McKinnon, Dorcas Meadowes, James Potter, Lily Potter,
Fabian Prewett, Gideon Prewett
Casualties of the second: Sirius Black, Remus Lupin,
Alastor Moody, Severus Snape, Nymphadora Tonks,
Emmeline Vance, Fred Weasley
Surviving members: Dedalus Diggle, Aberforth Dumbledore,
Elphias Doge, Arabella Figg, Mundungus Fletcher,
Rubeus Hagrid, Hestie Jones, Minerva McGonagall,
Sturgis Podmore, Kingsley Shacklebolt, Bill Weasley,
Charlie Weasley, George Weasley, Molly Weasley
Ex-member: Peter Pettigrew
Junior auxilliary: Harry, Ron, Hermione, and Dumbledore's Army
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. Possible grades are O(utstanding), E(xceeds Expectations), and A(cceptable) for passing and P(oor) and D(readful) for failure. The Weasley twins claim it's also possible to get T(roll). Actual O-levels are graded on an alphabetically arranged scale.
Ornithomancy (OotP ch. 25): Divination through birds. This was practiced in ancient Rome by the auspex ("bird-watcher") priest, from which we get auspice.
Oswald Fudge, Cornelius: Etym: From Old English oos "god" + weald "power".
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.