/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, see the instructions
    in the VSI OpenVMS System Manager's Manual, Volume 1: Essentials.