Not all educational materials available under a Creative Commons (CC) license are OER. Here is a handy graphic that shows which CC licenses are good for open educational resources, and which are not. 
"The easiest way to confirm that an education resource is an open education resource that provides you with the 5R permissions is to determine that the resource has been licensed under a Creative Commons license (or is in the public domain) that permits the creation of derivative works (i.e, allows revisions). Any of these CC licenses are OER-compatible: CC BY, CC BY-SA, CC BY-NC, or CC BY-NC-SA.
The two CC NoDerivatives (ND) licenses are not OER-compatible licenses because they do not allow the public to revise or remix an education resource. Because the ND licenses do not meet the 5Rs or any of the major OER definitions, the open education movement does not call ND-licensed education resources 'OER' " (Green, 2018).