Open Access (OA) refers to the removal of cost barriers, such as paywalls and high prices. The goal of open access is to make research, textbooks, and other materials open for all. Open access does not refer to copyright or the licensing of specific texts, but rather the initiatives themselves.
Open Educational Resources (OER) refer to works that are completely available for public use. The publisher of the text adds a license to the work, usually Creative Commons, which removes certain copyright protections. This allows people to "retain, reuse, revise, remix, and redistribute" the text without asking for permission. Making a resource into an OER typically allows people to read it online for free or low-cost.