COCI, the OpenCitations Index of Crossref open DOI-to-DOI references

COCI, the OpenCitations Index of Crossref open DOI-to-DOI references, is an RDF dataset containing details of all the citations that are specified by the open references to DOI-identified works present in Crossref, as of the latest COCI update. COCI does not index Crossref references that are not open, nor Crossref open references to entities that lack DOIs.

These citations are treated as first-class data entities, with accompanying properties including the citations timespan, modelled according to the data model described in the Open Citation Indexes page. For a full explanation of this, see our introductory blog post and following posts, particularly the one about COCI.

Currently, COCI contains:

COCI was first created and released: 4 June 2018

Most recent update of COCI; 4 June 2018

Future updates will occur at least every six months as more open DOI-to-DOI citations appear in Crossref.

Citation URLs

Each citation (i.e. an individual of the class cito:Citation) is identified by an URL structured as follows:[[OCI]].

Open Citation Identifiers

Each Open Citation Identifier [[OCI]] has a simple structure: the lower-case letters "oci" followed by a colon, followed by two numbers separated by a dash (e.g., in which the first number identifies the citing work and the second number identifies the cited work.

For citations in which the citing and cited works are identified by DOIs, which includes all the COCI citations, the OCI is created in the following manner, as explained more fully here. Each case-insensitive DOI is first normalized to lower case letters. Then, after omitting the initial doi:10. prefix, the alphanumeric string of the DOI is converted reversibly to a pure numerical string using the simple two-numeral lookup table for numerals, lower case letters and other characters presented at Finally, each converted numeral is prefixes by a 020, which indicates that Crossref is the supplier of the original metadata of the citation (as indicated at

OCIs can be resolved using the OpenCitations OCI Resolution Service.

Access to COCI data

All the data in COCI: