Yes, depending on how basic, or how long you want to work on it. Gary Kildall did CP-M. Tim Patterson did 86-DOS (later bought by Microsoft and turned into MS

Yes, depending on how basic, or how long you want to work on it. Gary Kildall did CP-M. Tim Patterson did 86-DOS (later bought by Microsoft and turned into MS-DOS). Terry Davis did TempleOS. And I'm sure there have been other, one-off operating systems written by single programers.