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

system operator
sysop
systems operator

    (Always abbreviated to "sysop") /sis'op/ A term
   used in the BBS world for the operator of a bulletin-board
   system (who is usually also the owner).

   A common neophyte mistake on FidoNet is to address a message
   to "sysop" in an international echo, thus sending it to
   hundreds of sysops around the world.

   Contrast: system administrator.

   (1999-10-20)