Open access (OA) is a method of sharing scholarship that is "digital, online, free of charge, and free of most copyright and licensing restrictions. What makes OA possible is the internet and the consent of the author or copyright-holder. OA is entirely compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance. Just as authors of journal articles donate their labor, so do most journal editors and referees participating in peer review. OA literature is not free to produce, even if it is less expensive to produce than conventionally published literature."
--From Open Access Overview, by Peter Suber
"OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge." 
The Open Education movement is built around the 5Rs of Openness: 
Open Access v. OER
Open Access is not the same as OER. While OER are created under an open license, Open Access materials are protected under traditional copyright and cannot be copied, shared, or remixed as can be done with OER. Open Access materials can be read online without a subscription or download. Open Access content cannot be embedded either; you can only link to these resources in the same way you would link to the library's databases.
Virginia Tech Provides an excellent example of the differences and nuances between OA and OER.
"How we (tend to) talk about Open Access and Open Educational Resources" by Anita R. Walz is licensed under CC by 3.0 US