Skip to main content
  • Subscribe by RSS
  • Ex Libris Knowledge Center

    Basic information on the OCLC control number

    • Product: Voyager
    • Product Version: insert product version
    • Relevant for Installation Type: Multi-Tenant Direct, Dedicated-Direct, Local, TotalCare



    Basic information on the OCLC control number.


    OCLC numbers are the foundation of bibliographic matching and de-duplication. Incorrectly formatted OCLC control numbers can cause issues with duplicate detection.  Consistency in OCLC number formatting is recommended (see "Summary of Suggested Formatting" below).

    On August 8, 2017, OCLC announced that WorldCat reached the 1 billion record mark. This marks the point predicted in 2013, when OCLC modified the format of OCLC control numbers to support a 10-digit and longer numbers.

    OCLC records follow MARC21 standards by storing the control number in the MARC 001 field. OCLC’s practice for record numbers provides a prefix, or “OCLC designation,” followed by eight or more digits.  Records with an “ocm” prefix cover records with eight digits; early record numbers of fewer than eight digits will include leading zeroes to reach eight digits.

    Records with an “ocn” prefix cover records having nine digit record numbers. Records with a control number of ten or more digits containing an “on” designation.

    Additionally, OCLC records include the MARC Organization Code for OCLC, “OCoLC,” in the 003 field. This information in a record indicates that a record was exported from WorldCat.

    Beginning in November 2006, OCLC records include an “automatically generated” control number in the 035 field of its records. This “second 035” does not include the prefix found in the 001 field. This number also lacks leading zeroes. The field contains the MARC organization code of “(OCoLC)” followed by the record number. This information establishes the identity of the “master record” in WorldCat, for a record in any other catalog.

    Some resource vendors (e.g., YBP, ProQuest) include an OCLC control number in the 035 of MARC records that they provide. Like OCLC’s 035, these include the “(OCoLC)” organization code followed by the record number, lacking the ocm, ocn, and on prefixes and leading zeroes.

    "Derived 035"

    When a library cataloger adds a MARC bibliographic record to a Voyager database, either via bulk import (i.e., batch loads) or via the cataloging client, Voyager concatenates the 001 (control number) and the 003 (control number identifier) fields from the original MARC record, and creates an 035$a field in the copy of the record that Voyager loads.  Because the OCLC control number and prefix (such as "on1234567890") and the "(OCoLC)" come from the 001 and 003 fields respectively, this data element is sometimes referred to as the "derived 035." Voyager indexes this 035 tag and uses it for duplicate detection.

    When viewed in the MARC record or in Voyager tables, the derived 035 appears as in the following examples.

    (OCoLC) with “ocm”+ 8 digits + blank space (numbers shorter than 8 digits have leading zeros; ‘^’ represents the blank space that is included at the end of numbers that are 8 digits or fewer):

    035 $a (OCoLC)ocm00456789^
    035 $a (OCoLC)ocm61367139^

     (OCoLC) with “ocn” + 9 digits:

    035 $a (OCoLC)ocn123456789

    (OCoLC) with “on” + 10 or more digits:

    035 $a (OCoLC)on1234567890
    035 $a (OCoLC)on876987654321
    "Second 035"

    With the OCLC control number expansion to 9 digits on 11/12/2006, OCLC also began automatically generating a system number field (i.e., 035) that contained an unprefixed OCLC control number. This is sometimes referred to as the "second 035".  For example:

    035 $a (OCoLC)456789
    035 $a (OCoLC)87654321
    035 $a (OCoLC)123456789
    035 $a (OCoLC)3987654321

    Note that these 035 fields lack the prefixes (“ocm”, “ocn”, “on”) found in the derived 035 fields, and that they do not include leading zeroes for numbers shorter than eight digits. When a library imports an OCLC record containing this format of the OCLC number, Voyager will retain this copy of the 035 along with the derived 035 from the 001 and 003 as noted above. Voyager will add this “second 035” to the indexes used for duplicate detection as well, if the number differs from what it finds in the derived 035: for instance, if the number lacks the leading zeros needed to have eight digits.


    When a bibliographic record enters a catalog, Voyager will attempt to normalize and add each 035 it finds into a left-anchored index for bib records. If the 035 contains the organization code OCoLC, or contains any of the OCLC designator prefixes (ocm, ocn, on), Voyager applies a specific normalization algorithm. Regardless of whether the 035 is "derived" or “second":

    1. For the 0350 index on the fully qualified record number,
      • The index entry will begin with “OCOLC” followed by a blank space;
      • Any OCLC designator (ocm, ocn, on) will be removed;
      • The numeric portion of the number will complete the index entry.
    2. For the 035A index on just the record number, the index entry will contain only the numeric portion of the record number.
    Summary of suggested formatting:

    (OCoLC)ocm01234567   (when the number of digits is less than 8, with leading zeroes so that the number of digits equals 8)

    (OCoLC)ocm12345678   (when the number of digits is 8)

    (OCoLC)ocn123456789   (when the number of digits is 9)

    (OCoLC)on1023456789   (when the number of digits is 10 or more)


    The above has been posted for informational purposes.  Please contact OCLC for definitive information about the 035 control number.  Voyager-L is also a useful resource for obtaining helpful information from colleagues at other Voyager sites.


    • Article last edited: 20-Jan-2021