Okawix the second off line program that supports openZIM. At this moment it is only the Linux client that has its initial code, but Okawix has the benefit of localisations in many languages.
When the software is stable for all the platforms, it will be interesting to see if the Wikimedia Foundation will acknowledge Okawix and give it the same status of that other open source product that supports openZIM, Kiwix.
In the end, the pudding is in the eating and when all the other things are equal, the difference will be in the quality and number of the localisations. Okawix has the advantage that is localised at translatewiki.net while Kiwix is localised using a text editor. Given that for many languages the localisation is not done from English and that there are no utilities that make the life of a localiser easy, Okawix has the best cards.
To be fair, we want a two horse race and handicaps are at the moment evenly divided. The race would be more interesting without handicaps; the Kiwix localisations deserve proper tooling and Okawix deserves support from the WMF.