From DOSpedia

DOS is group of closely-related PC-compatible operating systems which was initially introduced by IBM for the System/360 mainframe and later became common shorthand for the popular family of disk-based operating systems for x86-based IBM PC compatibles.

DOS primarily consists of Microsoft's MS-DOS and a rebranded IBM version under the name PC DOS, both of which were introduced in 1981.

Later compatible systems from other manufacturers are DR DOS (since 1988 by Digital Research, later sold to Novell then spun into Caldera, Lineo, and finally DeviceLogics).

ROM-DOS (since 1989 by Datalight), PTS-DOS (since 1993 by Paragon Technology and PhysTechSoft), Embedded DOS (by General Software), FreeDOS (1998), and RxDOS.