Open Educational Resources (OER) are teaching, learning, and research materials that have been released under intellectual property licenses that allows for both their free and open use and repurposing by others. OER can include course materials, textbooks, streaming videos, and any other resource that can be used to support open access to knowledge
Rice University has created a course that goes more in depth about OER. They have made it available as an OER through a PDF here. It covers topics such as evaluating an OER and how to create your own.
The terms “open content” and “open educational resources” describe any copyrightable work (traditionally excluding software, which is described by other terms like “open source”) that is either (1) in the public domain or (2) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities:
(Defining the "Open" in Open Content and Open Educational Resources was written by David Wiley and published freely under a Creative Commons Attribution 4.0 license at http://opencontent.org/definition/)
Open Access (OA) are educational materials that are available free online for anyone to use as is, but their repurposing by others may be restricted (OAs include government docs, articles from OA journals, reports from think tanks, etc.). OAs may not have the legal permissions to exercise all of the 5Rs.
While both types of materials are free online for anyone to use, only OER can be changed and then redistributed. OA generally must stay in their original format.
Below is a chart that goes more in detail about the differences.
"How we (tend to) talk about Open Access and Open Educational Resources" by Anita R. Walz is licensed under CC by 4.0 US