Wikipedia
OpenMG is a SDMI-compliant digital rights management system by Sony. It is designed for audio files in ATRAC3 format. The compliant software, e.g. Sony SonicStage, is usually capable of transcoding MP3 and wav files to OpenMG/ATRAC3. The file extensions OpenMG-encrypted files use are .omg and .oma.
There has already been at least one reported case when a security update of Windows broke OpenMG-compliant software. This issue was later resolved.
Sonicstage 3.4 includes an option to remove DRM from one's entire media library, allowing unrestricted use. However, this feature is disabled for copies without a license.
The compliant music organization systems, e.g. OpenMG Jukebox, work by "checking out" and "checking in" the files to/from portable players, keeping only one copy unlocked in order to hinder proliferation of copies.