The OS/2 Museum has found a working disc with PC DOS from July 1981 and got it working. It seems to be the oldest running PC operating system. The version includes some tools that were not included in the final release.
IBM PC DOS 1.0 appeared in August 1981, along with the IBM Personal Computer 5150, but when the OS/2 Museum booted the 160K disk it had received, it turned out that the operating system it contained was from early June 1981. The prerelease differs from the final software in several respects.
For example, unlike the later release, ASM.COM is present, the 8086 assembler that Seattle Computer Products, or SCP, used in its development. SCP was responsible for the development of 86-DOS. Microsoft purchased full rights to this software on July 27, 1981, and modified 86-DOS to release it as IBM PC DOS. In addition, the game Basic Space Wars was present in the prerelease.
The museum had some trouble getting the software up and running because the DOS version offered less stack space than later versions. It ran on an original IBM PC, but not always stable. According to the organization, it is an interesting museum piece and not the oldest DOS version, but the oldest operating system for the PC.