Metal Oxide Semiconductor - see MOSFET (metal oxide semiconductor field effect transistor).
Mit Out Sound or Mit Out Sprache cameras - MOS is a designation used on clapper boards to denote that no sound is being recorded in sync with the film. MOS cameras are film cameras without crystal-pulse capability, which is required to maintain a constant speed for sync sound. (Some British cinematographers maintain that "MOS" actually stands for "Motion Omit Sound." Some others say it stands for "Minus Optical Sound".)