Both Android and iOS take a very similar and sensible approach to data and application files when uninstalling an app - they simply remove EVERYTHING. This makes complete sense, as the user is uninstalling, so why should an uninstalled app still be using up precious storage space. OMN is no different, and we can't control this process.
A lot of people with apps when they hit a problem, simply uninstall, and then reinstall. Most of the time, this is in fact a pretty good strategy, in that everything gets removed, and then you get a new fresh copy of the app.
The issue with OMN is that if you have downloaded a lot of maps, the uninstall of the app will REMOVE the app and the maps. This means that after re-installation you will have to re-download your maps which can be a lengthy process.
Uninstall / Reinstall
If you only have a few small map areas, and the map downloads won't take very long, then feel free to uninstall/reinstall.
Settings->Reset will allow you to reset numerous parts of OMN without resorting to an uninstall/reinstall.
OMN will not start
Please see this link: OMN Fails to Start