Creative Commons defines OER 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– retaining, remixing, revising, reusing and redistributing the resources.
There is some disagreement on what makes a learning object, textbook or other educational resource "open." Some people equate "open" with "free", but there is more to it than that.
A truly open resource is one that you can not only access and use but one which grants you permission in perpetuity to use it in a variety of ways. The key elements that make a resource truly open are referred to as the 5Rs, and they include:
Portland Community College Library and Jen Klaudinyi for making their excellent OER guide shareable via the CC BY Creative Commons license. This guide borrows extensively from that work. https://guides.pcc.edu/oer