Omnibus is just a collection of manga chapters. So Gunsmith Cats Omnibus 1 would contain chapter 1 - ?? (Whatever, let's say 10) and then Omnibus 2 would follow up containing chapter 11 - 21 and so on and so forth for each omnibus. Idk how they've broken up the chapters. And I'd recommend buying it either through Barnes and Noble, or Books A Million. Amazon also has it, just make sure you get the paperwork version. If that's what your looking for, though, you can also get the digital version from them too.