There is only one file type that uses the .OMA extension:
Sony OpenMG Music File
Music file saved in Sony's proprietary ATRAC (Adaptive Transform Acoustic Coding) format; may contain DRM copy protection to prevent the file from being played on unauthorized computers or devices.
OMA files were used for distributing music through the Sony CONNECT Music Store, which was discontinued in March 2008.
Updated: June 23, 2014