I need to merge data from an OSM map with some other data (tiles) I get from a tileserver. I can import and display both layers well in QGIS and all zoom levels work fine. Now, I would like to export a region of this map as an .osm file so I can use it offline on my mobile phone (the app I am using on the phone can only load .osm files).
What I managed so far: I can export the merged maps from QGIS to an .mbtiles file. I can also extract that file to a folder structure or load the .mbtiles file as a single layer into QGIS again if that helps.
How can I get from the .mbtiles file to the .osm file, or more generally, how do I export the result of a merged OSM map with an tile-server map to single .osm file?
(additional info: I work with Ubuntu 14.04 and QGIS 2.18)