MOS signifies 'Mobile Operating System' (Russian: Моби́льная Операцио́нная Систе́ма, МОС). It was a Soviet clone of Unix in the 1980s. This operating system was commonly found on SM EVM minicomputers; it was also ported to ES EVM and Elbrus. MOS was also used in high-end PDP-11 clones.
There have been several modifications of MOS named MNOS, DEMOS, INMOS etc.
References
MOS (operating system) Wikipedia(Text) CC BY-SA