OERs are not copyright-free, but rather are generally available for use under Creative Commons or similar open licenses. These licenses grant permission to Retain, Reuse, Revise, Remix, and Redistribute resources.
There are six creative commons licenses, ranging from most open to least open. For detailed explanations of each license type, visit the Creative Commons website.
Image by Foter (CC BY SA) from https://www.wur.nl/en/article/What-are-Creative-Commons-licenses.htm