On the Origin of PCs is a prequel story featuring the heroes of the Order of the Stick webcomic. It is the first “print-only” OotS book, featuring material not available on the website.
As with the other print-only collections, this trade is in greyscale due to cost concerns.
On the Origin of PCs is 72 pages long, and features a short tale (or two) about each of the members of the OotS, as well as their formation as a team and first mission together. It’s a well done, humorous set of stories, but there’s nothing here vital to understanding the main story (nor anything all that important or illuminating really). But the background for Roy, Durkon and Haley is interesting, the trade as a whole is enjoyable, and it introduces some characters that would later appear in the “proper” trades (ie the thieves guild).
All in all this is a solid and enjoyable, albeit not totally necessary, addition to the OotS library. I wouldn’t call it “only for completists,” but you could skip it without losing much if you were so inclined.
While the volume number 0 is appropriate from a “in-comic” time perspective, it doesn’t tell you when you should be reading this volume (although the author somewhat does, in the introduction). I’d recommend reading it between volumes 3 and 4. This will prevent anything here from spoiling elements of the main story, and will allow you to get to know the characters before learning about their backstories.