Open educational resources (OER) are materials released under an "open" license which permits reuse according to the terms of the license. According to UNESCO, the United Nations Education, Scientific and Cultural Organization, 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.
As long as your use fits the license's requirements, you can use the work.
Frequently, OER are licensed under a "Creative Commons" license. For additional information about OER and Creative Commons, see: