It succeeded the Chippewa Operating System (shipped with earlier Control Data Corporation CDC 6000 series and 7600 computer systems), and was the Cray main OS until replaced by UNICOS in the late 1980s.
User jobs were submitted to COS via front-end computers via a high-speed channel interface, and so-called station software.
Front end stations were typically large IBM or Control Data mainframes.
Interactive use of COS was possible through the stations, but most users simply submitted batch jobs.
Magnetic tape datasets were also supported on Cray systems which were equipped with an I/O Subsystem.