Extracted from QTBUG-23412:
"The OSM/Mapnik plug-in could potentially utilize offline OSM data. The data is rather large on storage but when split into individual location (e.g. split by country) it is feasible to store map data on the local device (more details http://wiki.openstreetmap.org/wiki/Downloading_data)"

I currently use data extracted this way and display it using libosmscout library. Unfortunately performance is not the best and it is widgets-based, so it introduces new requirements for QML-based applications. I'd love to see OSM offline plugin in Qt.