harry potter - Can Occlumency be used upon Dementors?

Occlumency is generally intended to guard against Legilimency--the penetration of the mind by an outside source. The dementors are not performing Legilimency as such. They are not reading anyone's mind, trying to identify their emotions, or attempting to force a particular thought on them. Yes, they force people to relive their negative emotions, but this is a side effect of their overall desire to feed on positive emotion. The dementors want food, not a way inside someone else's mind, which is the goal of a Legilimens. A Patronus Charm succeeds by providing the dementors with something else to feed on.

If they dementors do not recognize human emotion in something, they know that it cannot offer them a food source, and so they will generally leave it alone. Sirius took advantage of this tendency by using his Animagus form in Azkaban. The only way Occlumency might succeed in warding off a dementor is if an Occlumens manages to block all his human emotions to the point that the dementor cannot sense any of them. But I am inclined to think that such a total block would be difficult, if not impossible. Patronus Charms are complicated enough, considering that Hogwarts does not teach them until students are at the N.E.W.T. level. And few students bother to become Anamagi, as the national register reflects: only the most advanced Hogwarts pupils seem to be offered this opportunity to learn how (i.e., Professor McGonagall). Occlumency is not taught at Hogwarts at all. For most wizards, Patronus Charms are the only familiar option.

So I would answer this question with a very qualified yes. If a given Occlumens can block his or her emotions to the point that his or her emotions are not recognizable as human, then Occlumency could work against dementors. Whether anyone, even Snape, is skilled enough to manage this feat is another matter. And again, Occlumency was invented as a counter to Legilimency, not as a protection against dementors. Bellatrix Lestrange was skilled enough at Occlumency to instruct Draco, but the books imply that she was affected by the Dementors in Azkaban, just like all the other prisoners. Although Occlumency might be able to ward off dementors in theory, as a practical matter, using it in this non-traditional way would require an impossibly high level of skill.