According to Creative Commons, "OER are teaching, learning, and research materials that are either (a) in the public domain or (b) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities– retaining, remixing, revising, reusing and redistributing the resources."
"5 R's of Openness" by David Wiley is licenced under CC BY 4.0
For information about properly attributing OER in your online courses, please refer to Creative Commons' Best practices for attribution.