Open educational resources (OER) are copyrightable works that can be used for educational purposes and are either in the public domain or under a copyright license that provides free and perpetual permission to retain, revise, remix, reuse, and redistribute. The term "Open Educational Resources" was coined at a 2002 Forum on Open Courseware organized by the United Nations Educational, Cultural and Scientific Organization (UNESCO). OER can include full courses, course materials, modules, textbooks, videos, tests, and any other copyrightable physical or digital tools or materials used to support access to knowledge. The most commonly used legal schema for granting an open license is provided by Creative Commons. Instructional techniques that utilize or rely on OER are generally classified as Open Educational Pedagogy, Open Educational Practices, or OER-Enabled Pedagogy.

The concept of OER exists only in the legal context of copyright law, where creators can grant permissions to users through a license. The most commonly accepted set of permissions is the 5R Activities, which include permission to retain, revise, remix, reuse, and redistribute copyrightable works. Creators often reserve some rights and requirements on the use of their works that are less than full copyright but more than no rights reserved. The six Creative Commons copyright licenses are used in the legal creation of OER, as they allow for all of the 5R Activities.

OER is the subject of much academic research, with varying goals and approaches. A large corpus of research is built on the COUP Framework, which explores the impact of OER through the lenses of Cost, Outcomes, Uses, and Perceptions. Several meta-analyses of OER research have been published, exploring the overall impact of OER across various metrics and in various contexts. Policy related to OER has been implemented throughout the world at many different levels of governance, incentivizing the adoption and use of OER by educators. In 2019, UNESCO adopted a Recommendation on OER that requires member states to monitor policies and mechanisms related to OER using a combination of quantitative and qualitative approaches.

Related terms include Open Pedagogy, Open Education, Open Educational Practices, Open Licensing, Open Pedagogy, Open Textbooks. Community artifacts are also related to OER, as they can be used to support access to knowledge.