SPECIAL NOTE: 86—DOS is not related to the popular CP/M operating system of Digital Research. Disk directory formatting and space allocation are completely different and incompatible. 86—DOS does, however, provide a utility called RDCPM which will transfer files from CP/M disks to 86—DOS disks. Further, operating system calls and calling conventions have been provided which make possible automatic translation of Z80 programs written for CP/M into 8086 programs that run under 86—DOS.
86 DOS Version 0.1 C Serial # 11 (ORIGINAL DISK) (archive.org) This is the precursor to MS-DOS and is likely the oldest known version to survive. (I had previously uploaded Version 0.34, which was at the time thought to hold that honor.)
86-DOS (Wikipedia) 86-DOS (known internally as QDOS, for Quick and Dirty Operating System) is a discontinued operating system developed and marketed by Seattle Computer Products (SCP) for its Intel 8086-based computer kit.