Open Educational Resources (OER) are educational and research materials which are in the public domain or released by their creators with open licenses such that you are allowed free use of the materials. OER may include full courses, textbooks, assignments and exams, or any supplemental instructional material such as lecture notes, syllabi, images, slide decks, videos or software. Generally OER are provided free of cost.
David Wiley's "5R Permissions" are often referenced as the standard for what defines OER.
According to the 5Rs, an OER allows you to:
Adapted from The Access Compromise and the 5th R (Wiley, 2014)
No. OER are OA but not all OA are OER. OER are a subset of OA.
OA materials are free to use but not all OA materials are specifically licensed to allow the 5R's of OER. You may not alter or remix OA materials unless it has been specifically licensed as OER. For example, some scholarly journal articles are published as OA; it is free to read them but you are not permitted to alter the content. OER materials, however, do allow you to do so.
Thanks to our colleagues at Merritt College Library for this information.