Open Educational Resources are educational materials and resources that are publicly accessible meaning that they are openly available for anyone to use and under some licenses to re-mix, improve and redistribute.
You may already be familiar with open access journals and books. These materials are not "free". Someone had to create them and costs were borne by some party. But the author or publisher licensed the content so that any member of the public may access it and possibly re-use or re-format it.
OER is similar in that the individuals who create these resources are licensing the content so that it is publicly accessible and may be re-purposed by others for educational applications.
OER is a movement in education that seeks to counter costly, commercially produced learning content, typically textbooks, with publicly accessible content that is licensed so that it can be freely distributed and shared. Here is an official definition from the Organization for Economic Co-operation and Development:
Educators and learners as well as learning institutions are driving its development. OER provides an alternative to the rising costs of education. It alleviates the burden of student debt while providing opportunities to students who might not otherwise be able to afford or access materials. In response to the effectiveness of OER compared to traditional commercial textbooks, research now indicates that OER are equally or more effective.
OER provides an opportunity to try new ways of teaching and learning, many of which are more collaborative and participatory.
Educators across the K-16 spectrum are taking advantage of OER to (1) move away from traditional textbooks and (2) improve learning by introducing students to more varied and current learning content. In higher education, faculty are adopting OER as a way to save their students money but also increase the likelihood that students will acquire and read learning content.
If you include free online courses, another type of learning content included in the definition of OER, almost any citizen of the planet who is taking advantage of a free online course, a Kahn Academy Video, an educational video - they are all using OER.
Who isn't using OER. Too often faculty at higher education institutions are not aware of the existence of OER. Academic librarians are joining forces to help create more awareness at their institutions.