“By 'open access' to the 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 Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities
OA and OER are cousins and share the fact that they are openly available to make sharing knowledge easier.. However, while being closely related, they were created for different purposes. OA are created to provide open access to resources (mainly for scholars and researchers) but do not address copyright. OER are resource used for educational purposes and are licensed with a creative commons license or are in the public domain.
Green vs. Gold
Green Open Access refers to an author sharing their own work, commonly via an institutional repository or OA website, after publishing in a traditional outlet. Sometimes only a pre-print version may be shared, or there may be an embargo period before the author can share the accepted manuscript.
Gold Open Access is when the published version of an article is available, without restrictions, to the public. Some journals are entirely Gold OA, others are blended Gold/Green, and some have contracts that allow Green OA.