Open Access (OA) stands for unrestricted access and unrestricted reuse.
"PLOS applies the Creative Commons Attribution (CC BY) license to works we publish. This license was developed to facilitate Open Access—namely, free immediate access to, and unrestricted reuse of, original works of all types. Under this license, authors agree to make articles legally available for reuse, without permission or fees, for virtually any purpose. Anyone may copy, distribute or reuse these articles, as long as the author and original source are properly cited. Additionally, the journal platform that PLOS uses to publish research articles is Open Source."
Excerpt from Public Library of Science (PLOS).
From Open Educational Resources (OER) libguide by Heather Blicher
Be aware that the open access journal collections listed on this guide page may have SOME open access articles in addition to those articles that are fully copyrighted. That is, the journals in these collections may be themselves open access, but you may have to use a filter to find the open access articles within the journals AND check the actual article for CC BY licensing information. Here is a graph that explains the range of access that may be found within the continuum of closed access to open access publications (click graphic to see larger pdf version).
Remember, if you want open access ARTICLES, look for a filter (like a check box) for open access so you can refine your search results within these collections.