This question came up in an edit. There are, unfortunately, many examples of both in the DB, so I’d like to see if there’s a consensus on how it should be.
Original edit with discussion: Edit #94015750 - MusicBrainz
For soundtracks, when you see “Original Soundtrack”/“Music from the Motion Picture”/etc. phrase on the cover, is this a subtitle or ETI?
My opinion, and I think it follows the Soundtrack guidelines, is that it is a subtitle, and therefore to be in the RG title following a colon:
Constantine: Original Motion Picture Score
or
Constantine (Original Motion Picture Score)