There are 2 file types use the .OSB file extension:
1. Osu! Storyboard File
An OSB file contains a storyboard created by osu!, a game of rhythm where players tap visual cues in sync with music. It stores the directory path of an image, typically a .JPG or .PNG file, that is displayed with an .OSZ Beatmap file during gameplay. It also includes plain text information that describes when and where the image is displayed in the background depending on how the player performs.
OSB files are located in the same "Beatmaps" folder of the osu! game installation as the OSZ file they are associated with. Typically, the installation directory is C:\Program Files\osu!\Beatmaps\, but you may have to create the directory manually.
Updated: August 24, 2018
2. OpenStreetMap Binary Map File
File used by Merkaartor, an OpenStreetMap (OSM) editor; saves the XML data from an .OSM file in a binary format; allows more compact file sizes and faster manipulation, but is more difficult to read, since it is not stored in plain text.
Updated: January 28, 2011