According to UNESCO, Open Educational Resources are "learning, teaching and research materials in any format and medium that reside in the public domain or are under copyright that have been released under an open license, that permit no-cost access, re-use, re-purpose, adaptation and redistribution by others." OER freely grant permission to engage in the "5R" activities as described by David Wiley:
OER include not only open textbooks, but also lesson plans, lectures, videos, quizzes, and other educational materials.
Graphics are from "Making the Case for Open Educational Resources" by C. Edward Watson and Rachel Rush-Marlowe, which is licensed under CC BY-NC-SA 4.0