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"It's a secret society," said Hermione quickly. "Dumbledore's in charge, he founded it. It's the people who fought against You-Know-Who last time."
That is what Hermione said of them in book 5, which is correct. At the moment, not much is known of the OotP. This is because it was just recently released in Book 5, and as it is secret, not much is known elsewise.

Essentials
Leader: Albus Dumbledore
Purpose: Fighting Lord Voldemort and his supporters(death eaters)
Headquarters: 12 Grimmauld Place
Members: Overage Wizards(out of school)

Members
Remus Lupin*
Alastor "Mad-Eye" Moody- ex-auror
Arthur Weasley- MoM (Misuse of Muggle Artifacts)*
Molly Weasley*
Bill Weasley- Gringotts
Charlie Weasley- Dragons?
Nymphadora Tonks- auror
Kingsley Shacklebolt- MoM (Sirius Black search)
Elphias Doge
Deladus Diggle
Emmeline Vance
Hestia Jones
Albus Dumbledore- Hogwarts Headmaster*
Aberforth Dumbledore
Arabella Figg-squib
Severus Snape- Hogwarts teacher(Potions), spy*
Minerva McGonagall- Hogwarts teacher(Transfiguration)*
Mundungus Fletcher- criminal
Rubeus Hagrid- Hogwarts teacher(Care of Magical Creatures), gamekeeper*
(deceased,etc.)
Sturgis Podmore- Azkaban
Sirius Black- dead*
Alice Longbottom- insane
Frank Longbottom- insane
Lily Potter- dead*
James Potter- dead*
Marlene McKinnon- dead
Benjy Fenwick- dead
Edgar Bones- dead
Caradic Dearborn- dead
Gideon Prewett- dead
Fabian Prewett- dead
Dorcas Meadowes- dead
Peter Pettigrew- death eater

Quotes(thanks to MuggleNet)
"I will only truly have left this school none here are loyal to me... Help will always be given at Hogwarts to those who ask for it."-Dumbledore

"If you made a better rat than a human, that's not much to boast about."-Sirius Black

"To the well organized mind, death is but the next great adventure."-Albus Dumbledore

"Never trust anything that can think for itself, if you can't see where it keeps its brain."-Arthur Weasley

"It is our choices Harry, that show what we truly are, far more than our abilities."-Dumbledore

"Fear of a name increases fear of a thing itself." -Dumbledore

"When a wizard goes over to the dark side there's nothin', and no one matters to 'em anymore." -Hagrid

"If you want to know what a man's like, take a good look at how he treats his inferiors, not his equals." -Sirius Black

"Remember Cedric. Remember, if the time should come when you have to make a choice between what is right and what is easy, remember what happened to a boy who was good, and kind, and brave, because he strayed across the path of Lord Voldemort. Remember Cedric Diggory." -Dumbledore

"I don't need a cloak to become invisible." -Dumbledore

"Your mother died to save you. If there is one thing Voldemort can't understand, its love." -Dumbledore

"Scars can come in handy. I have one myself above my left knee that is a perfect map of the London Underground." -Dumbledore

"I seem to remember telling you both that I would have to expel you if you broke any more school rules," said Dumbledore. Ron opened his mouth in horror. "Which goes to show that the best of us must sometimes eat our words," Dumbledore went on, smiling.

"And Potter -- do try and win, won't you? Or we'll be out of the running for the eighth year in a row, as Professor Snape was kind enough to remind me only last night..." -Prof. McGonagall

"What was there to be gained by fighting the most evil wizard who has ever existed? Only innocent lives, Peter." -Sirius Black

"There's enough filth on my robes without you touching them." -Sirius Black

"Not at all up to your usual standard, Hermione," he said. "Only one out of three, I'm afraid. I have not been helping Sirius get into the castle and I certainly don't want Harry dead...but I won't deny that I am a werewolf." -Lupin

"You are truly your father's son, Harry..." -Sirius Black

"'Course Dumbledore trusts you. He's a trusting man, isn't he? Believes in second chances. But me -- I say there are spots that don't come off, Snape. Spots that never come off, know what I mean?" -Moody

"As much money and life as you could want! The two things human beings would choose above all. the trouble is, humans do have a knack of choosing precisely those things that are worst for them." -Dumbledore

Gallery (coming soon)