"The Dark Lord, for instance, almost always knows when somebody is lying to him. Only those skilled at Occlumency are able to shut down those feelings and memories that contradict the lie, and so can utter falsehoods in his presence without detection."
-- Severus Snape, speaking mainly of himself as an Occlumens (OP24)
A practitioner of Occlumency is referred to as an Occlumens. Occlumency is the closing of one’s mind to outside intrusion through use of the Legilimens spell.
- Severus Snape - Harry was often aware of Snape's penetrating gaze, but what he didn't realize was how much those same eyes kept hidden. However, we have a clue during the first Potions Class: "Snape's eyes cold and empty and made you think of dark tunnels" (PS8); clearly Snape had shut down his emotions on that day, in the same way he was always able to close his mind to the Dark Lord, keeping his loyalty to Dumbledore and love for Lily Potter a deep dark secret even while protecting Harry (DH33). It was a mental and emotional juggling act, and an Oscar-worthy performance that should put him in the Occlumens Hall of Fame, if there is such a place in the Wizarding World.
- Tom Riddle / Voldemort - At Hogwarts Tom Riddle was somehow able to hide his evil deeds from Dumbledore, a known Legilimens. Tom opened the Chamber of Secrets releasing the Basilisk that killed Moaning Myrtle, and turned his Diary into a horcrux, before murdering his father and grandparents, and Dumbledore knew little or nothing of this until years later (CS18,HBP23). As Lord Voldemort he was able to close his mind (at times) from Harry's view in spite of the Horcrux connection between them (HBP4).
- Albus Dumbledore - The Headmaster never gave away his plans and thoughts to others, so his Occlumency skills were excellent. Because of that skill, almost everything he did surprised the Wizarding World. However his Legilimency skills were not as strong, since other Occlumens were able to hide nefarious plans right under his nose, not just Tom Riddle but also . . .
- Peter Pettigrew - The Dark Lord's "Most Faithful Servant" was able to become an illegal Animagus at Hogwarts, join the Order of the Phoenix and do the Fidelius Charm with the Potters even while selling them out to Voldemort. Then he faked his own death, killed 20 Muggles, framed Sirius Black, and hid at Hogwarts as a rat. He fooled his best friends and everyone else in the Wizarding World, but especially Dumbledore, a Legilimens who never seems to have suspected his two-faced nature (PA18,PA19, OP9). The only one he could never fool was Lord Voldemort, who accused him of lying almost constantly while they stayed together at the Riddle House. Even as "Baby-mort" the Dark Lord could see right through Peter's flattery and lies (GF1).
- Barty Crouch Jr. - This Death Eater also faked his own death while at Azkaban with the help of his parents, then used Polyjuice Potion to masquerade as one of Dumbledore's best friends, Alastor "Mad-Eye" Moody. He got away with teaching Defense Against the Dark Arts for a year, even while taunting Snape - a Legilimens who was suspicious but unsure of what Fake Moody was doing - then kidnapped Harry during the Triwizard Tournament so the Dark Lord could use Harry's blood to rise again (GF35). Clearly the disguise and glass eyeball of the disfigured Mad-Eye helped him avoid eye contact that might otherwise have blown his cover.
- Draco Malfoy - As a Junior Death Eater, Draco was tasked with finding a way to get Death Eaters into Hogwarts and then kill Albus Dumbledore. As he slowly fell apart under the pressure, he also took time for Occlumency lessons with his Aunt Bellatrix - and we can only imagine how pleasant that was, having the sadistic Bella attack his mind over and over with the "Legilimens!" spell (HBP15). But the practice paid off, and even Professor Snape couldn't find out exactly what Draco was up to until it happened, and Snape was forced to kill Dumbledore on top of the Astronomy Tower when Draco failed (HBP27). But Draco didn't fail at being an Occlumens, a skill even Hermione couldn't match.
- Bellatrix Lestrange - Since Draco's aunt was his teacher for Occlumency we can probably assume she knew how to protect her own secrets. She was trusted by the Dark Lord with the plan to lure Harry to the Department of Mysteries, as well as the fact that the Hufflepuff Cup was hidden in her vault at Gringotts (DH35). The only secret she might have been able to keep from Voldemort was that she acted as binder during the Unbreakable Vow between her sister Narcissa and Severus Snape, since he never seemed to know about it. (HBP2).
occulto - (Latin) to hide, conceal, cover (the English word 'occlude' comes from this root, as well as the modern astronomy terms about one celestial body hiding another from view) mens - (Latin) mind
Other characters who might be Occlumens, but not confirmed in the canon:
- The Other Marauders: James Potter, Sirius Black, Remus Lupin - Like Peter Pettigrew, James and Sirius were also unregistered Animagi and Dumbledore gave them credit: "An extraordinary achievement -- not least, keeping it quiet from me" (PA22). Remus Lupin also knew, but kept the secret during their Hogwarts years and beyond, after Sirius was in Azkaban, James was dead, and he believed Peter was dead. When Snape confronted Lupin about the Marauder's Map, "An odd, closed expression appeared on Lupin's face," which could have been Occlumency (PA14). Lupin seems to have Legilimens ability also, facing Sirius again and "staring at Black so intently it seemed he was trying to read his mind" (PA17). These were probably useful skills for a werewolf who tried to avoid trouble.
- Narcissa Malfoy - She was almost hysterical at Spinner's End when asking Snape to help Draco, but in the presence of the Dark Lord she knew how to keep her cool. At Malfoy Manor, Narcissa tried not to draw much attention from Voldemort, keeping a "deadpan stare" at the wall of the dining room, and avoiding eye contact as much as possible -- all signs of Occlumency (DH1). When Harry faced Voldemort in the Forbidden Forest and they both fell during their duel, it was Narcissa who lied that Harry was dead so she could get back to Hogwarts and find Draco. (DH36) There are several explanations for why Voldemort believed her lie: Harry's sacrifice possibly kept Voldemort's Legilimens from working, or Narcissa was too far away and out of eye contact, or perhaps it was simply that the Dark Lord never understood motherly love. Because of other explanations, we can't say for sure that she was an Occlumens, but it's highly likely.
- Lucius Malfoy - It was his plan to put Tom Riddle's Diary in Ginny Weasley's cauldron at Flourish and Blotts, and reopen the Chamber of Secrets. Apparently Dumbledore never knew what was going on even with petrified children falling down all over the place. However, after Harry destroyed the Diary, Dumbledore confronted Lucius by staring into his eyes and asking questions, while Lucius's face "turned masklike" as if he was practicing Occlumency (CS18). By then, Dumbledore had figured out the truth, so it's possible that Lucius understood Occlumency in theory, but wasn't very good at it. It's probable that he never knew about Snape's Unbreakable Vow with Narcissa either, since he was in jail when it happened and there is no instance of him talking about it. That may be because he wouldn't have approved, but more likely Narcissa knew he couldn't keep it a secret (HBP2).
- Dolores Umbridge - When she sent Dementors to attack Harry in Little Whinging, she kept it a secret from Cornelius Fudge, Madam Bones and others in the Ministry, then later with Dumbledore - no small feat. She also lied about the origin of Slytherin's Locket to other wizards at the Ministry, including Yaxley, a Death Eater. Whether that falls into Occlumens territory is up for debate, but she certainly has all the traits of a well-practiced liar (OP32). Her biography states that while her father was a janitor in the Ministry of Magic, she often lied and told people he was a member of the Wizengamot (Pm).