OpenCitations is an independent not-for-profit infrastructure organisation for open scholarship dedicated to the publication of open bibliographic and citation data by the use of Semantic Web technologies. 

OpenCitations fully espouses the founding principles of Open Science. It complies with the FAIR data principles by Force11 that data should be findable, accessible, interoperable and re-usable, and it complies with the recommendations of I4OC that citation data in particular should be structured, separable, and open. OpenCitations has recently published a formal definition of an Open Citation and has launched a system for globally unique and persistent identifiers (PIDs) for bibliographic citations – Open Citation Identifiers (OCIs).

The role of OpenCitations in OpenAIRE-Nexus is to provide open bibliographic citations, and interconnect and integrate (and vice versa) functionalities with the OpenAIRE Research Graph and more OpenAIRE-Nexus services (such as EpiSciences, OpenAIRE MONITOR) the core component of OpenAIRE infrastructure and services and of the EOSC Resource Catalogue.

OpenCitations is dedicated to the publication of open bibliographic and citation data, thereby providing a disruptive alternative to traditional proprietary citation indexes. In particular, OpenCitations provides:

• a data model: the OpenCitations Data Model (based on theSPAR Ontologies); 
• bibliographic and citation data (CC0): OpenCitationsCorpus, COCI, CROCI, CCC;
• software: in our GitHub repository, released with open source licences;
• online services: REST APIs, SPARQL endpoints, dumps and interfaces.

Open Citations main database COCI (the OpenCitations Index of Crossref open DOI-to-DOI citations; http://opencitations.net/index/coci/) now contains more than one billion citation links derived from open references within Crossref, including the references within journal articles published by Elsevier and the American Chemical Society recently been made openly available at Crossref. The citations are published under the CC0 public domain waiver and can be accessed via a flexible API using DOIs and OCIs (Open Citation Identifiers).

The citations available in COCI are treated as first-class data entities, with accompanying properties including the citations timespan, modelled according to the OpenCitations Data Model.

The Graffoo diagram of the main ontological entities described in the OCDM.

OpenCitations enables:

1. Fairness, since it avoids institutions and independent scholars having to pay tens of thousands of dollars annually (that most of them cannot afford!) for commercial access to their own scholarly data;
2. Reuse, since there are no licence restrictions and all data, are provided under CC0, so users can re-publish and reuse for any purpose the citation data that OpenCitations provides;
3. Research assessment, since we provide open data usable for national and international research evaluation exercises, enabling one to make such activities transparent and reproducible;
4. Community governance, since the community is directly involved in the evolution of the infrastructure.

Using OpenCitations is beneficial because it:

• Provides (without charge under open licences) open metadata with a scope, depth, accuracy and provenance, as a disruptive alternative to traditional proprietary citation indexes;
• Supports stakeholder communities of libraries (authors, researchers, students, institutional administrators) to monitor research;
• Enables reproducibility in bibliometrics research;
• Tracks the scholarly productivity and influence of members of academic community;
• Exploits free availability of citation data to build new applications and visualisations;
• Guarantees easy access to the citation data through different points of access, depending on the particular need.

Programmatic access to OpenCitations data may be obtained either via SPARQL endpoints or via our REST APIs. In addition, OpenCitations data can be downloaded from dumps - available in CSV, Scholix, and RDF - made periodically and stored on Figshare, so as to support large-scale analyses using the whole content of the data sets. The services OpenCitations makes available for programmatic access of its data are used in several tools and platforms including Citation Gecko, DBLP, OpenAccess Helper, VOSviewer and the Internet Policy Review Journal, and its dumps have been ingested in several services (including The Lens and Inciteful) and used in several scholarly publications.

One of OpenCitations’ main priorities is to keep its services, software and data always without charge under open licences for fostering maximum reuse. The data held in any of the OpenCitations datasets are made freely available under a Creative Commons public domain dedication (CC0).

The text of the web pages that comprise the OpenCitations website is made freely available under a Creative Commons Attribution 4.0 International Public License.

The software developed by OpenCitations for implementing all the services is made freely available on GitHub under the ISC License.

Technical requirements:

• For using the services for programmatic access to the data (i.e. SPARQL endpoints and REST APIs), it is necessary to have a working internet connection and to specify the queries in a compliant way with the Web standards adopted (i.e. SPARQL and the HTTP protocol).
• For downloading the dumps, it is necessary to have enough available space on the hard drive so as to correctly store and uncompress the archives – information about the size of the archives are available in the OpenCitations download page.