According to SPARC (the Scholarly Publishing and Academic Resources Coalition), OER are "teaching, learning, and research resources that are free of cost and access barriers, and which also carry legal permission for open use." Creative Commons licenses are often used to grant permission for re-use (see more on that below). Examples of OER include:
SPARC describes Open Access as follows: "the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment." Creative Commons licenses are often used to grant permission for re-use (see more on that below). Most OA materials are published in a final iteration, and can be shared freely but not modified.
Open Education Resources (OER) differ in that while they are also Open Access and freely available, they are also intended to be remixed, built upon, and further customized to suit the needs of different audiences within the education and learning environment. OER also extend to curriculum materials in addition to books and articles.
In short, just because it's online, freely available, and/or Open Access, doesn't necessarily mean that it is an Open Education Resource. However, all Open Education Resources are also Open Access.