The Omnibus series is a collection of trade paperbacks that were first released by Dark Horse Comics starting in 2006. The goal of the project is to collect comics that were formerly near-impossible/difficult to find. Dark Horse plans to release two or three per year.
28.06.2006 - Further information was given by Randy Stradley (Dark Horse Editor) on the Dark Horse Message Boards:
Eventually, there will probably be an Omnibus for every series (or for groups of stories taking place in the same time period). Each Omnibus volume will contain the equivalent of three to five 4-issue trades. But, since we currently have something like 150 trades (and more on the way), and since we're limited to two to three Omnibusses per year, it will be a while before all of the existing trade paperbacks are "omnibussed", and even then it will be an ongoing program.