Open Educational Resources (OER) are materials for learning, teaching and research in any format and medium that:

• are in the public domain or copyrighted
• issued under an open licence that permits free access, re-use, re-purposing, adaptation and redistribution by others.
  
  

OER can include: full courses, course modules, open textbooks, syllabi, lectures, student handbooks, lecture notes, homework, quizzes, laboratory and classroom activities, pedagogical materials, videos, interactive materials such as simulations or role-plays, assessment tools and many other resources, software, programmes (including mobile applications) and any other educationally useful materials.

UNESCO's recommendation on Open Educational Resources (OER) - Recommendation on Open Educational Resources (OER), UNESCO, 25 November 2019.
UNESCO's mandate in OER, UNESCO, April 21, 2022.

OPEN TEXTBOOKS AND BOOKS

MIT OpenCourseWare is an initiative of the Massachusetts Institute of Technology (MIT) to publish open access educational materials in its undergraduate and graduate courses. Course materials are available under a Creative Commons Attribution-NonCommercial-ShareAlike license in the humanities, social sciences, engineering, environmental sciences, computer science, physics and mathematics, economics, biology and law.

Directory of Open Access Books - access to 4,589 academic peer-reviewed books from 155 publishers from all fields of study.

OER Commons - public digital library of open educational resources from different fields of knowledge: applied sciences, life sciences, social sciences, physical and mathematical sciences, humanities, law, business, education.

OpenEd at BCcampus is a Canadian government project focused on providing students with open access textbooks and other educational materials in such disciplines as engineering, computing, humanities, law, social sciences, physical and mathematical sciences, health and medicine, business and economics, tourism, the environment, and higher education.

Open Education North Carolina Textbooks is an initiative to provide open textbooks for courses taught at North Carolina universities. Covers: STEM education, humanities and social sciences, business and economics, statistics, biology, mathematics, chemistry, tourism, education.

Open Culture: 200 Free Textbooks access to 200 open textbooks from renowned scholars and over 1,700 free online courses from the world's leading universities from such disciplines as business and management, biology, chemistry, computer science and information systems, earth science, economics and finance, education, engineering, humanities, social sciences and law.

Open Textbook Library - a collection of more than 1,088 open textbooks covering computer science, humanities, social sciences, natural sciences, engineering, law, medicine, mathematics, business and management, journalism and communications, and education.

LibreTexts: Free the Textbook - access to 398 open textbooks, 154 courses in the following fields of study: biology, chemistry, business, engineering, statistics, earth sciences, physical and mathematical sciences, humanities, social sciences, education.

OpenStax College is an educational initiative of Rice University. Free access to an online form of peer-reviewed textbooks in a variety of fields: mathematics, medicine, biology, business, humanities and sciences, and higher education.

EdEra Books is a social project of interactive educational literature. It is developed by a team of young scientists, programmers, designers and experts. The online books at EdEra consist of illustrated texts with integrated videos and tests to check the knowledge acquired. They allow to connect different methods of information perception and increase the effectiveness of learning. Best of all, they are available online for free, 24/7.

Open Textbook Initiative from the American Institute of Mathematics seeks to encourage the adoption of open source and open access mathematics textbooks. 

Open Educational Resources Search Index (OERSI) is an initiative of the German National Library of Science and Technology. A searchable index of open educational resources containing more than 43,000 open educational materials. Covers: Engineering, Humanities, Social Sciences, Natural Sciences, Mathematics, Law, Economics, Medicine, Sports.

Twillo is an initiative of the German National Library of Science and Technology. A portal of free and open-licensed educational materials available for use in learning and teaching from the fields of engineering, mathematics, natural sciences, medicine and health, and the humanities.