Source : Free On-Line Dictionary of Computing

control-O
     
         {ASCII} character 15 (SI).
     
        The character used on some {operating system}s 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)