The Jargon File (version 4.4.7, 29 Dec 2003):

control-O
 vi.

    ?Stop talking.? From the character used on some operating systems to abort
    output but allow the program to keep on running. Generally means that you
    are not interested in hearing anything more from that person, at least on
    that topic; a standard response to someone who is flaming. Considered
    silly. Compare control-S.


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

control-O

    ASCII character 15 (SI).

   The character used on some operating systems to abort output
   but allow the program to keep on running.  The name "SI" comes
   from its use on some terminals to "shift in" an alternative
   character set.  "SO" is Control-N.

   Compare control-S.

   [Jargon File]

   (1995-03-16)