An expression typically used by trance junkies, when they hear a decent track.
Sometimes it can be used in the ironic sense, and needn't be associated with a track of outstanding musical merit. For example, upon hearing 'Tiesto - Traffic', while hardly a work of genius, omg choon would still be an appropriate reaction, because, well...it's 'Traffic' innit.
See: OMG I DIE, omgtijs, etc.
<Armin van Buuren> That was new the new track from Push!!
<dave> omg choon!!