"a standalone novel and also book one of the new Cainesville series, which continues with Visions".
So I expected a kind of prequel to the series, maybe one that had different main characters compared to the upcoming books in the series. But that's not the case, the main characters of Omens are the main characters of Visions, the ending of Omens is not an ending that stands for itself, you need to read on with the series because otherwise you will have no idea what the hell happened. So no, in my opinion Omens doesn't work as a standalone novel, not at all.
Besides this misleading info I so far enjoy the story a lot, I'm on page 85 and excited to read on. I want to know more about what happened in the past with Olivia's parents and want to know how she will deal with being the daughter of serial killers.