It lists stories such as The Other Side, Displaced Aggression, and Haunted Holidays.
Are these stories collected within the regular Vol collections? How do these stories relate to the main series that is currently ending? In other words, if collecting the whole series, will I need this Omnibus in addition to the volumes?
Those all happened before the ongoing series. “The Other Side” was IDW’s first mini-series (and was…not well recieved). “Displaced Aggression” came next, and then there were four holiday-themed one-shots, which were collected into the “Haunted Holidays” trade paperback. One of the “Haunted Holidays” one-shots, and a story that only appeared in the collections, is illustrated by Dan Schoening, artist of the ongoing series. The Omnibus collects all of that material in one place.
Next in the line-up would be the Ghostbusters: Infestation two-issue mini-series. It was written by Erik Burnham, and it serves as a bit of set-up for the ongoing series.
You don’t really need any of those to enjoy the ongoing, though as mentioned the Infestation story does serve as some set-up for the ongoing series, and one of the Haunted Holidays stories (“Tainted Love”) introduces a character that plays a significant role in recent events (I’ll say no more to keep from spoilers)
Of the numbered trade paperbacks, 1-4 are Vol.1 of the ongoing series (issues 1-16); all sixteen issues are also collected in the Total Containment hardcover.
Then trade paperbacks 5-9 (the last of which is not released yet) collect Vol.1 (issues 1-20; issues 1-4 were titled New Ghostbusters).
And almost inevitably, there will probably be a trade collecting the TMNT/Ghostbusters crossover which hasn’t started yet.