Differences between OA and OER:
Open Access refers to removing barriers such as "paywalls". Open Access initiatives seek to make research articles and other works easy to find and read, for free. It does not address copyright, but rather the methods of funding and accessing the research or other works.
Open Educational Resources are works that copyright owners have "opened" by adding a Creative Commons or other License that removes some copyright restrictions. The ideal is to allow others to "retain, reuse, revise, remix, and redistribute" without needing to ask for permission, as long as the work is attributed to the copyright owner, giving credit where it is due.
Copyright owners can choose which copyright restrictions to remove by selecting the appropriate Creative Commons License.
After a Creative Commons License is attached to a work, it can not be revoked for that copy of the work...but the copyright owner still retains all rights and privileges of copyright for their work.
There is now a serious gap between students, scientists, and other researchers that come from different economic environments.
The Open Educational Resources (OER) movement promotes the creation of "Open Access" works that are free to use. This includes promoting the use of high quality textbooks that are free to students, reducing an important barrier for under-resourced students. This reduces the gap in access to knowledge between people of different economic circumstances.
The Open Access movement argues that users shouldn’t have to pay twice for access to the research supported by public funds. They call for free “public access to publicly funded research.”
Even in the U.S., many libraries cannot afford the subscriptions to elite journals. Many useful articles are never read by people who are not associated with a richly subsidized institution. Peer-reviewed Open Access Journals are making it easier for scientists to access and share information. This also reduces the gap in access to knowledge between people of different economic circumstances.