VMS Help SET, VOLUME, Qualifiers, /STRUCTURE_LEVEL  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /STRUCTURE_LEVEL=n

    Sets the structure level of the volume. Use the value 5 to set
    the volume to ODS-5.

    When the /STRUCTURE_LEVEL qualifier is used with other
    qualifiers, the volume structure level is set prior to the other
    qualifiers being performed.

    You cannot use the SET VOLUME command to change a volume from
    ODS-5 to ODS-2. To reset a volume to ODS-2, refer to instructions
    in the OpenVMS Guide to Extended File Specifications.

                                   NOTE

       Compaq does not support creating the system disk as (or
       changing it to) an ODS-5 volume.

    Refer to the OpenVMS Guide to Extended File Specifications for
    further information.