os --- option source

        default range           syntax
           1,$       os[pma | ftn | f77 | s | f]

        This option sets other options for  case, tabs, etc.  for one
        of  the three programming languages listed.  The option "oss"
        is the same as "ospma" and the option "osf" is the same thing
        as "osftn" (the  corresponding command line options are "-ss"
        and "-sf").  If no argument is specified the options affected
        by this command revert to their default value(s).

        For general information about the options command, type "ho".