Steve Wozniak did DOS 3.3 for the Apple // and the SOS for the Apple ///. He open-sourced a lot of his work for others to learn from.
DOS 3.3 was mostly Shepardson Microsystems Paul Laughton and Randy Wigginton from Apple. Woz supplied the RWTS routines for reading / writing sectors.
Linked from that page is this very nice description of how the Apple 2 disk operating system was written. In 7 weeks, on punch cards and paper tape!
Today I learned that, like Windows NT, the first version of Apple DOS was numbered 3.1.