Open Citation Identifier Resolution Service


The Open Citation Identifier (OCI) is a globally unique persistent identifier for bibliographic citations, created and maintained by OpenCitations, and this page provides a resolution service that takes an OCI and returns information about that citation.

Each OCI has a simple structure: the lower-case letters “oci” followed by a colon, followed by two numbers separated by a dash. For example, oci:1-18 and oci:2544384-7295288 are both valid OCIs.

The first number is the identifier for the citing bibliographic resource, while the second number is the identifier for the cited bibliographic resource. Within the OCC, these bibliographic resource identifiers are unique, and every citation recorded in the OpenCitations Corpus (OCC) has an OCI. OCIs have also been created for open citations within other bibliographic databases where citing and cited works bear unique identifiers of the same type, and the OCIs from these sources have been used to populate OpenCitations Indexes.

With the exception of the internal identifiers within the OpenCitations Corpus for works ingested prior to February 2018, all bibliographic resource identifiers have a supplier prefix - a short numerical string delimited by zeros that indicates the supplier of the identifier. For example 010 indicates that Wikidata is the supplier. Thus oci:01027931310-01022252312 represents a citation between two bibliographic resources (i.e. wd:Q27931310 and wd:Q22252312) whose metadata was in both cases supplied by Wikidata. Similarly, 020 indicates Crossref is the source of the DOIs used to create the OCI. Thus oci:02001010806360107050663080702026306630509-02001010806360107050663080702026305630301 represents a citation between two bibliographic resources identified in Crossref by DOIs (i.e. and, whose metadata can be retrieved in both cases by means of the Crossref API, if desired. A list of existing OCI bibliographic resource identifier prefixes and their meanings can be found in the following table.

PrefixSupplierIdentifier type
no prefixOpenCitations CorpusOpenCitations Corpus Internal Identifier (for works ingested prior to Feb 2018)
010WikidataWikidata Identifier (QID)
020Crossref Digital Object Identifier (DOI)

While the coverage of OCIs among all bibliographic citations is at present limited, bear in mind that the coverage of DOIs (digital object identifiers) to identify published works was also limited following its launch in 2000. As the coverage of the OpenCitations Indexes, the OpenCitations Corpus, and other similar resources expands, the number of OCIs in use will increase proportionately.