You could try
here for the latest nightly version. ( v2015.11-658 today ).
A slight correction, KOReader is not an OS. In fact it runs under the same Kobo Linux that the stock reading programs and the Nickel UI run under. (For Kobo devices, of course. For Kindles, Pocketbooks, and Android devices, their respective OS is used.) The KOReader project includes its own User Interface (UI), written in the LUA scripting language, that can call on both the Coolreader3 Engine and a MuPDF based engine.
CBZ files that are processed by the Kindle Comic Converter work fine in KOReader on my Kobo AuraHD.
There is a KOReader forum here now, btw.
Luck;
Ken