The first live-action musical film to have a commercially issued soundtrack album was MGM's 1946 film biography of Show Boat composer Jerome Kern, Till the Clouds Roll By. (Snow White was also a musical film, but an animated one.)
In reality, however, soundtrack producers remain ambiguous about this distinction, and titles in which the music on the album does lock to picture may be labeled as OCS and music from an album that does not lock to picture may be referred to as OMPS.