.ogm files are actually ogg media container files; they usually have a DivX or XviD video stream, with Ogg Voribis audio, and maybe subtitles as well. VLC seems to have no trouble at all playing them, but I'm not so sure the QT component will help; it might play the audio, but unless it tells quicktime how to handle the OGM container, I don't think you'll be able to see the video.
Can someone correct me on that?