There is only one file type that uses the .OSM extension:
OpenStreetMap Map File
An OSM file contains street map information in the OpenStreetMap (OSM) format. It saves XML-formatted data in the form of "nodes" (points), "ways" (connections), and "relations" (street and object properties, such as tags).
OSM files can be used for creating small street maps, or they can scale to store streets worldwide. The OpenStreetMap project maintains a freely available file named Planet.osm that is an ever-growing database of the world's streets.
The OpenStreetMap project uses a Web interface for displaying and exporting OSM data. There are also many FTP sites that host the data for download.
Updated: October 24, 2019