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:0301-03018 and oci:0302544384-0307295288 are both valid OCIs for citations defined within the OpenCitations Corpus.

The first number is the identifier for the citing bibliographic resource, while the second number is the identifier for the cited bibliographic resource. Each identifier is composed of two parts, a prefix that identifies the supplier database from which the citation is recorded, and a number is a unique identifier for a citing or cited entity within that database. Within the OpenCitations Corpus (OCC), all internal bibliographic resource identifiers are unique, and every citation recorded in the 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.

Each prefix is a short numerical string delimited by zeros that indicates the supplier database from which the citation is recorded, and in which the identifier is unique. For example 030 is the OpenCitations Corpus (OCC) prefix, while 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 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. In the Crossref example, the DOIs have been converted into purely numerical strings using a simple two-numeral look-up table, as explained in the OCI definition document.

A list of existing OCI bibliographic resource identifier prefixes and their meanings can be found in the following table.

PrefixSupplierIdentifier typeExample
010WikidataWikidata Identifier (QID)oci:01027931310-01022252312
020CrossrefDigital Object Identifier (DOI)oci:02001010806360107050663080702026306630509-02001010806360107050663080702026305630301
030OpenCitations CorpusOpenCitations Corpus Internal Identifieroci:0301-03018
040DryadDigital Object Identifier (DOI)oci:040050006013613273410133708070900-04003070302361019113701000000040801
050CROCIDigital Object Identifier (DOI)oci:05001000106361937321411281422370200010237000837000001-050010008073602000009020002

With the use of OCIs to create COCI, the OpenCitations Index of open Crossref DOI-to-DOI references, the coverage of OCIs now extends to ~450 million bibliographic citations. As the coverage of the OpenCitations Indexes, the OpenCitations Corpus, and other similar resources expands, this number will rise still further.