V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

OS9
       Operating System - 9 (Microware, OS), "OS-9"


The Free On-line Dictionary of Computing (30 December 2018):

OS-9

    A real-time operating system written by
   Microware Corporation.  The original version was written
   about 1978 for the Motorola 6809 and has since been ported
   to the Motorola 68000, Intel 80386, Intel 486, and the
   PowerPC.  The kernel of OS-9 is ROMable, modular, with a
   unified file system, allowing it to easily be scaled up or
   down as required.

   FAQ (http://os9archive.rtsi.com/os9faq.html).

   User Group (http://cs.wisc.edu/~pruyne/os9ugfaq.html).

   Usenet newsgroup: news:comp.os.os9.

   (1996-04-03)