MOS is a term that says the film has no soundtrack. There a myth that it means Mit Out Sound, credited to Lubitsch but I was a sound engineer early on and it meant (then) Mask Optical Sound (although other definitions abound). Before the advent of a tape track, sound was optical, variable area or variable density. There was a danger of an overexposed soundtrack spilling into the picture area of a print, so the sound was masked when it wasn’t needed, at least it was then.  In the early fifties magnetic sound began to appear in films but optical sound still remained and was improved. With the new digital technology, all those systems are obsolete.