OS/2 was designed to be the replacement of DOS and Windows, by IBM ... and ... Microsoft. It had an interesting history, and up until they were ready to release OS/2 NT (http://www.os2museum.com/wp/nt-and-os2/) even Microsoft believed in it. However history was not kind at that moment, and MS and IBM split, causing OS/2 NT being repurposed as Windows NT, and the rest of the story is well known.
NT microkernel had support for separate subsystems (OS/2, Windows, and Unix). Even until Windows NT 4, it was able to run (command line) OS/2 applications natively, in addition to Windows. (This is more or less how the Linux applications run today on the recent "Creator's Update" stuff. They have a separate kernel API for the Linux subsystem).
However NT took on, OS/2 did not. Mostly due to technical reasons: it was fast, but not stable on most devices, except for a small approved subset, had a single message queue for the system, whereas NT had true multi-tasking, and would not run 32 bit Windows applications, only 16 bit ones. I was sad that this happened, but given many good alternatives today, like Haiku OS optimized for multimedia, Linux for everything else, and yes Windows for desktop, it might not be a big loss.