From UNESCO: Open Educational Resources (OER) 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.
How do I know if something is an OER?
Any resource you come across that is OER will have a Creative Commons (CC) license somewhere within the resource (bottom of the webpage, under the image, etc.). This graphic illustrates the different kinds of CC licenses you may see, and what they mean.
If you are unsure if something qualifies as an OER, ask a librarian at the desk for help!