Open Access Resources are research materials have been made available to the general public, free of charge: data and datasets, books and articles, including scholarly research articles. This is not the same thing as public domain works, or those items that have fallen out of copyright protection; open access authors have chosen to share freely, so anyone can use the works legally without permission or fees. Isn't that nice of them?
Public Library of Science, 2010, https://flic.kr/p/8L4egi