Open Access research is digital, online, available at no cost to the public, and free of most licensing restrictions.
The Budapest Open Access Initiative states, "There are many degrees and kinds of wider and easier access to this literature. By 'open access' to this literature, we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited."
OA and OER are closely related, but differ in the purpose for which they were created. OA is a broader, more inclusive term that generally refers to research publications for scholars and researchers to share their work under an open license. OA publications tend to be journal articles for which authors have no interest or intention on making money, but want to share their work for its impact on the field.
OER are teaching and learning materials developed for and licensed under open licenses. The goal of OER is not only to save costs and improve access, but to encourage experimentation, innovation, and collaboration to create effective educational resources.
In higher ed, there is a clear overlap as the set of teaching and learning materials requires the use of research publications in addition to textbooks and other secondary resources.