An oclcnum is a sequential accession number assigned by OCLC as bibliographic
records are entered into OCLC WorldCat (the OCLC Online Union Catalog).
Numbers are not re-used, but when records are merged, the old number is kept
as a cross-reference to the new record. oclcnum identifiers are text
representations of integers, e.g., 53393962.  They are composed entirely of
digits.  There is no limit on the number of potential oclcnum identifiers.  As
of November 2003 there are over 52 million records in WorldCat.

OCLC numbers are often specified in the 035 tag (System Control Number) in
MARC records.  In these cases the control numbers are preceded with the MARC
code of the organization originating the system control number,
e.g.,(OCoLC)814782.  The leading MARC code, (OCoLC)in this example, should be
stripped off to create a properly formed oclcnum identifier.  The OCLC number
is displayed by some software programs as ocmnnnnn, i.e., ocm53393962. The
leading "ocm" should be stripped off to create a properly formed oclcnum
identifier.