An OSB file is a game file used by osu!, a free-to-play online rhythm game where you click, slide or hold on to beats in a song. It contains information about a beatmap, which includes the song title, artist, and maps of beats to the rhythm of the song. These files are stored in a plain text format.
Opening OSB Files With osu!
By far the most common use of OSB files is within the osu! game. When you download a beatmap from the osu! website, it typically comes in the form of a .osz file, which is a compressed format containing both the song data and .osb file for the beatmap. To use the .osb file, all you have to do is open the .osz file with osu!, and the game takes care of the rest.
OSB File in External Applications
In addition to osu!, there are a few other applications which can open OSB files, typically for the purpose of beatmap edit or modding. These include Nrwl Nx, a set of extensible dev tools for monorepos, and Notepad++, a versatile text editor which can open virtually any text file. Simply download and install the application, then try to open the .osb file as you would any other file.
OSB File Important Information
An important thing to note about OSB files is they are not meant to be opened manually. The .osb file is typically automatically loaded by osu! when opening the corresponding .osz file. Manual opening of the .osb file and editing it without a concrete understanding of its format can result in unpredictable in-game behaviors and is thus not recommended. Always make sure you have up-to-date versions of osu! or any other related software to ensure the best compatibility with .osb files.