OER stands for Open Educational Resources. Creative Commons defines OERs as:
"teaching, learning, and research materials that are either (a) in the public domain or (b) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities (Retain, Reuse, Revise, Remix, and Redistribute)."
Find out more about OERs in this Library Guide on Open Education Resources.
Here's a selection of OERs from the following OER sites: