Emergency Shutdown System Maintenance Override Switch (MOS) is a built-in switch provided in the DCS to enable maintenance of trip transmitters/system so as to avoid inadvertent trip of the plant/unit.
Normally the overridden trips gets back to normal after a period of time say 8 hours.
Maintenance overrides are used to disable inputs or outputs from a safety related system for maintenance activities, to avoid trip actions caused by spurious signals or logic tests
In general, we must put a lot of restrictions about using MOS. This could damage the plant while we are trying to maintain the availability.