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We know Severus Snape was "an accomplished Occlumens." We also see that Voldemort, who was a powerful Legilimens, was unable to detect when Snape was lying to him.
Does this mean that Voldemort could not detect when he was being lied to because of Snape's Occlumency? Would he know that Severus was protecting his thoughts from being read, had Voldemort tried Legilimency on him?
If Voldemort had ever known -- or even suspected -- that one of his inner circle was using Occlumency against him, I don't think that person would have lived for very long. Therefore, either:
I lean toward the latter. I'm sure a clumsy beginner's attempt at Occlumency would be easy to spot, but perhaps a skilled-enough Occlumens is able to hide the fact that they're hiding anything -- to leave parts of their mind open while blocking other parts, and leave even a powerful Legilimens like Voldemort none the wiser.
Personally, though, I think that Snape had additional help, beyond even his skill in Occlumency: (spoiler for book 7)
Snape's love for Lily.
Love is the antithesis of everything Voldemort ever stood for; he never understood it, and it actually caused him pain when he possessed Harry in OotP. I think a skilled Occlumens like Snape could turn that to his advantage in steering Voldemort away from any thoughts relating to protecting Harry.
Consider also that Dumbledore is also a skilled Legilimens. Snape is effectively playing a double agent, openly meeting both Dumbledore and the Dark Lord often. If the Dark Lord ever questions him about Occlumency, he can claim that he has to hide his true allegiance very deep in his mind so that Dumbledore cannot see it, so he has locked his mind with Occlumency so strongly that it would be impractical to open it up for the Dark Lord to read it.
Voldemort, to the end, believed Snape was a very loyal servant of his. Even when he thought he’d need to kill him to master the Elder Wand, he actually regretted it, because he was killing someone he considered a valuable and faithful Death Eater. Though he felt no emotions over Snape, he thought him worthy enough of regret - if he thought Snape was disloyal, he surely wouldn’t regret killing him.
“Snape did not speak.
‘Perhaps you already know it? You are a clever man, after all, Severus. You have been a good and faithful servant, and I regret what must happen.’
‘My Lord –”
- Harry Potter and the Deathly Hallows, Chapter 32 (The Elder Wand)
When Harry tells him that Snape was actually loyal to Dumbledore instead, Voldemort is at first dismissive of the idea.
“Snape’s Patronus was a doe,’ said Harry, ‘the same as my mother’s, because he loved her for nearly all of his life, from the time when they were children. You should have realised,’ he said, as he saw Voldemort’s nostrils flare, ‘he asked you to spare her life, didn’t he?’
‘He desired her, that was all,’ sneered Voldemort, ‘but when she had gone, he agreed that there were other women, and of purer blood, worthier of him –”
- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)
If he knew when Snape was lying to him, then Voldemort would have surely doubted his loyalty more, since his lies were told to hide his true loyalty. He may have known that Snape was using Occlumency to hide his thoughts, but he couldn’t have known for certain that Snape was lying and maintain such a high level of belief in his loyalty.
It’s never explicitly stated, but Voldemort probably didn’t even know Snape was using Occlumency on him. If he had known, it seems likely that he would, at least, trust Snape less than he did. It’s also possible that Snape gave him a plausible reason why he’d use Occlumency all the time and never be willing to “turn it off” momentarily for Voldemort to see his entire mind, but this too seems to be something that would make Voldemort less certain in his trust of Snape.
