Unprotected (without DRM) OMA files in Sony ATROC format can be converted to standard MP3 with the utility provided by Sony that can be download on their website.
As for the DRM protected ones, there is really no way how to convert them, or even play them on other machine than for which they were purchased.
If you have Sony SonicStage program, you may want to try to burn the music as standard CD, then rip the CD back to MP3., which will essentially achieve oma to mp3 conversion.