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.

Some bibliographic resource identifiers have a supplier prefix - a short numerical string delimited by zeros that indicates the supplier of the metadata. 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 that COCI is the supplier. Thus oci:02001010806360107050663080702026306630509-02001010806360107050663080702026305630301 represents a citation between two bibliographic resources (i.e. and, whose metadata can be retrieved in both cases by menas of the Crossref API, if needed. A list of existing bibliographic resource identifier prefixes and their meanings can be found in the following table.

no prefixOpenCitations Corpus

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.