Open Educational Resources (OER) are teaching, learning and research materials in any medium that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others.
Definition from Creative Commons. Other definitions can be found here: https://wiki.creativecommons.org/wiki/What_is_OER%3F
You don't have to go full OER
OER are great! So are library resources and traditional textbooks and materials! You can use them in combination, though you probably can't adapt paid resources. Check out this sample of library resources with potential for your classes. Looking for something? Contact your librarian!
Creative Commons (CC)
Creative Commons (CC) is a non-profit organization dedicated to expanding access to creative works. CC licenses allow creators to clearly communicate rights relating to the use and reuse of their work. Most, but not all, CC licenses are suitable for OER.
Creative Commons is a great resource for creators and users of content. All Creative Commons licenses require users to attribute works to their author. Learn more about attribution under Creative Commons licenses here.
"Creative Commons." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 9 May 2018. en.wikipedia.org/wiki/Creative_Commons. Accessed 14 May 2018.
Creative Commons. "What we do: What is Creative Commons?" creativecommons.org/about. Accessed 14 May 2018.