Open Educational Resources (OER) are teaching, learning, and research materials that reside in the public domain or have been released under an open license that permits their free use and re-purposing by others.
Examples of OER include:
Opencontent.org states that true OER are free to access and allow for the 5Rs of open content:
Making Open Educational Resources: A Guide for Students by Students by Ashlyne O'Neil; Jykee Pavo; Mikayla Bornais; Tariq Al-Rfouh; Chris Nardone; Elijah Annoh-Waithe; Lawrence Villacorte; Lorenzo Pernasilici; Marianne Kantati; Mitchel Macmillan; Mohamed Eldabagh; Norman Ha; Devin Wacheski; Anas Al-Chalabi; Dave Cormier; Brandon Mailloux; Ghanem Ghanem; Kamaal Kusow; Kristen Swiatoschik; Patrick Carnevale; Rana Kilani; Steven Shlimoon; and Zain Raza is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.