There is only one file type that uses the .OSK extension:
osu! Skin File
Skin file used by osu!, a game of rhythm where players tap visual cues in sync with music; contains graphics and sounds that alter the appearance and sounds of the game; used to customize hit circles, the score display, the slider type, the cursor, and hit bursts.
You can install custom skins by placing them into the \Skins\ directory of the osu! game installation (typically C:\Program Files\osu!\Skins\, but you may have to create the directory manually). If you downloaded an archive (e.g., a .RAR or .ZIP file), you will need to first decompress the enclosed folder and then place the folder in the \Skins\ directory.
Updated: August 4, 2012