According to the Community College Consortium for Open Educational Resources, "Open licensing does not replace copyright. Open licenses work with copyright to promote shared use. This changes the copyright from “all rights reserved” to “some rights reserved.”
"Creative Commons: A User Guide" by Simone Aliprandi, Creative Commons Attirubtion-ShareAlike 3.0 Unported.
There are many materials students and faculty can freely access online, including library resources. Only materials that have an open license or are in the public domain are OER. This is because OER must be open as defined by the 5Rs:
The 5 Rs of Open