The Cray Operating System (COS) was Cray Research's proprietary operating system for its Cray-1 (1976) and Cray X-MP supercomputers, and those platforms' main OS until relieved by UniCOS in 1984. COS was delivered with Cray Assembler Language (CAL) and Cray FORTRAN (CFT).

See also: Cray Time Sharing System