OER are either in the public domain or have an open license, which gives users rights to do more than traditional copyright would allow. These open licenses are assigned by the creator of the work and define what is and is not permissible to do with the work. Before using a resource, it's important to understand what rights or restrictions you may have in using the work.
Creative Commons licenses are used frequently in the OER community. "Creative Commons licenses provide an easy way to manage the copyright terms that attach automatically to all creative material under copyright. [These] licenses allow that material to be shared and reused under terms that are flexible and legally sound. Creative Commons offers a core suit of six copyright licenses" (from creativecommons.org).
Descriptions of the licenses, below, are quoted from https://creativecommons.org/licenses/ and used with permission, thanks to a Creative Commons Attribution 4.0 International license.