In 1991, Apple released Macintosh System 7.0.
System 7 was too large to work comfortably from a floppy disk so it was the first version of the Macintosh operating system to require a hard drive. It was also the first Apple operating system to be available on CD although it shipped on floppy disks initially.
Read more about System 7 at http://en.wikipedia.org/wiki/System_7