As you said, the Original Soundtrack is the CD that is being sold as the soundtrack of this game. The Complete Soundtrack contains a total of 52 tracks, which are the ingame music. Without having heard either of them closely, my guess would be that the Original Soundtrack tracks are those originally composed, which were later adapted for use ingame. So even if the Complete has some of the music from the Original, the one in Original will be different, even if only slightly
But after all, you get both soundtracks, and if you want to mix a cd, just pick the ones you like.