Pathfinder products are published under several different lines of books. There's the Roleplaying Game line, which consists of the hardcover rulebooks. There's the Pathfinder Campaign Setting and Pathfinder Player Companion lines, and of course, Pathfinder Adventure Path amongst others. Each line gives an indication of what people can expect from the books published in it—rules material, adventures, etc.
However, within the lines, there are sometimes smaller series—not generally officially marked as such, but with naming patterns to indicate them. There are the Revisited and Unleashed books in the Campaign Setting, or the Blood of... books in the Player Companions. One of the smaller groups like these is the Origins books, with Mythic Origins, Advanced Class Origins, and now Occult Origins. These books are companions to rulebooks (Mythic Adventures, Advanced Class Guide, and Occult Adventures respectively), with each one introducing the concepts of their respective rulebooks in the world of Golarion and, primarily, offering lots of new player options. I've commented in my reviews of the last two that Origins is a bit of a misleading name, as they don't really discuss the origins of the new material or even the origins of characters using that material. That said, Occult Origins is better in this regard and actually does briefly discuss how characters become some of the occult classes.
In fact, Occult Origins is definitely the best of the Origins books to date. Paizo has refined the series with each successive book. Occult Origins is a book of mostly “crunch” (i.e. mechanical rules options for characters), but it is the best kind of crunch—the kind that supports the flavour of the setting as well as giving characters fun new options. The material in this book is full of flavour that both expands the world of Golarion and expands our understanding of it. And this only serves to enhance the gaming experience.
From the moment you open this book, you get a feel for its flavour. The inside front cover contains an overview of the “Planes of Occult Power”. These are the standard Inner Planes of the setting, but from the esoteric point of view introduced in Occult Adventures. There is a diagram of the Inner Planes and a sidebar detailing how occult things like spirits, phantoms, and the Cosmic Fire relate to these planes.
The book proper begins with an introduction broadly explaining how the occult and the occult classes fit into Golarion. Although it is a brief discussion that doesn't have the space to go into a lot of detail, it nonetheless firmly grounds the occult into the setting. What I particularly like about this section and the book as a whole is that it maintains the sense of mystery and rarity that is an important part of the occult as introduced in Occult Adventures. Mythic Origins strangely manages to make mythic characters feel commonplace, but Occult Origins does not fall into that trap. Even though players have the option to be any class (subject to GM discretion, of course) and could even make entire parties of occult characters, the book places them into the world in such a way as to maintain their unusualness.
After the introduction, roughly half the book focuses on the new classes introduced in Occult Adventures, offering new options for each. As in Occult Adventures, the kineticist gets the longest section, at four pages, while the other five classes get two pages each. Golarion has long had cultures that don't break the elements up into the standard four air, earth, fire, and water, and Occult Origins acknowledges this by adding two new elemental specialisations for kineticists: void and wood. Void is similar, though not identical, to aether (the fifth element in Occult Adventures). Whereas aether is force, void is the substance of the Negative Energy Plane. Void kineticists are called chaokineticists. Phytokintecists (wood) are primarily found in Tian Xia, but can be found in other areas of the world, too. The majority of the kineticist section is taken up by descriptions of talents for the two elements, and of the six classes, kineticists actually get the least “fluff” description. Nevertheless, the Golarion flavour is still quite strong.
The section on mediums provides a lot of background information about their place in the world. In particular, we learn a fair amount about Geb and Nex, two nations that have not received a lot of detail to date. We are introduced to the Nexian channeller, a medium archetype that lets the medium channel the spirit of Nex (the founder of the nation of Nex) instead of one of the six standard spirits. The Nex spirit is a modified archmage spirit. Despite only being two pages in length, there's a surprising amount of information in this section, and the Nexian channeller is a perfect example of how to blend mechanical options with world flavour.
The section on mesmerists focuses on the role of this class in the nations of Nidal and Cheliax. There are some new tricks and masterful tricks, as well as a few new spells. There are also “devilbane gazes”, which are a form of bold stare improvements developed by Chelish mesmerists.
The two pages dedicated to the occultist discuss how someone on Golarion becomes an occultist and introduce the reliquarian, an archetype devoted to a god and that can cast divine instead of psychic spells. The section also contains a selection of “sacred implements” that occultists can choose in place of an implement school. Each sacred implement is specific to one of Golarion's gods. Unfortunately, there's not enough room to include an implement for every god, so there are only seven here (one of which is Aroden—possible because sacred implements contain residual energy left behind by their associated gods at some point in the past). However, these seven provide a good enough introduction to sacred implements that GMs can easily create additional ones for the other gods as they are needed.
The section on psychics discusses their place on Golarion, but actually goes beyond this with a short description of the role psychics play on other worlds like Akiton and Castrovel. The section also contains some new amplifications and two new psychic disciplines: enlightenment and rebirth.
The section on spiritualists discusses how phantoms are viewed by other people in the world—particularly how they are viewed by the church of Pharasma. Since phantoms are the spirits of the dead, many people naturally see them as undead abominations and they see spiritualists as either necromancers or people restraining souls from their rightful journey into the afterlife. It makes for an interesting but difficult place that spiritualists have in the world. This section contains an archetype (the fated guide), a new emotional focus (remorse) and a couple of new feats.
After the six occult classes, Occult Origins offers six new archetypes for other classes. These focus on adding a few occult abilities to non-occult classes. I find these archetypes to be the least interesting material in the book. They don't quite have as much world flavour as other things in the book (although a couple of them, like the Harrowed Society student, an arcanist archetype, have more flavour than the others).
Following the archetypes is a selection of new occult feats. Many of these tie into occult skill unlocks, which constitute one of my least favourite parts of Occult Adventures (see my review of that book for my reasons), but these feats do have a lot of colour to them. None of them are particularly Golarion-specific, but they maintain the occult flavour encountered throughout the the rest of the book.
The next section includes some new occult rituals, each tied into the legends and mysteries of various parts of the campaign setting. I really like the idea of rituals—primitive magic that anyone can potentially use—and Golarion's history is rife with inspiration for rituals. Invoking the hero-god is a ritual from lost Sarkoris, while Peacock Spirit's tranquil roar invokes one of the ancient gods of Thassilon.
The final two sections focus on spells, the first on Occult Spells and the last on Psychic Spells. It's not entirely clear what the difference between these two are, other than none of the psychic spells are available to non-occult classes while the occult spells are. Given that Occult Adventures doesn't distinguish between spells in this way, I'm not sure why this book does. It actually makes finding specific spells more difficult (not to mention that there were already some new spells earlier in the book) as you don't immediately know which section to look in. That aside, there are some interesting spells here, particularly subjective reality, which allows the caster to convince herself that a real target is actually an illusion. The caster can then treat the target as an illusion, even being able to walk through the target.
On the whole, Occult Origins is definitely the best of the Origins books. It offers a great selection of new options for characters, particularly the occult classes, but what's best about these new options is that they are steeped in world flavour. This book succeeds admirably in making the material from Occult Adventures fit seamlessly into the Golarion setting.