VideoLAN and DO

I don't think the VideoLAN ActiveX control works with any of the existing Opus plugins.

Someone could write a plugin that integrated VLC into Opus but nobody has so far.

If you install FFDShow and the splitters described here then you should be able to play just about anything (including MOV, MKV, FLV) in Opus.

You should already be able to play MP3 files in Opus via the Windows Media Player ActiveX control and Opus's ActiveX plugin. You probably just need to disable Opus's MP3 tag editing plugin so that the files drop through to the ActiveX plugin.