Open access works “are digital, online, free of charge, and free of most copyright and licensing restrictions" (Suber, 2004-2015); the purpose of “open access” is to make information freely available to find and read. Open Access most often is used in in reference to works of scholarly research, like peer-reviewed journal articles or books. The author of an Open Access work retains copyright and determines how their resources can be used, but makes it freely available to access.
Open Educational Resources, take Open Access a step further. OER refers to works that copyright owners have "opened" by adding a Creative Commons or other license that describes the copyright owner's conditions for use of their work. The idea 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. OER can also be journals and books, but often extend further to other types of teaching and learning materials (e.g. courses, course materials, interactives/simulations).
It is important to note that there is a lot of overlap between OA and OER. Some open access works use open licenses, like the Creative Commons licenses. The Directory of Open Access Journals, for example, has journals that use a variety of licenses, but the majority fall under the CC By license. It is important to refer to the journal's license, however, to ensure you are using the article properly.
Works that are in the Public Domain can be copied with no need to obtain permission or pay royalties. They are effectively OER, as well. A work may be part of the Public Domain for one of the following reasons:
Resources dealing with Public Domain: