There are different ways in which Journal Articles are made available as Open Access
An open database of 21,648,962 free scholarly articles. They harvest Open Access content from over 50,000 publishers and repositories, and make it easy to find, track, and use. Unpaywall is run by Impactstory, a nonprofit dedicated to making scholarship more accessible to everyone.
An online directory that indexes and provides access to high quality, open access, peer-reviewed journals.
A directory of academic peer-reviewed books from many publishers.
Provides access to academic open access repositories from around the world.
Promotes the development of open access by providing timely information about the growth and status of repositories throughout the world
This is the most common way of locating open access materials. As shown in the example below there will often be two links, one to the freely-available PDF version in an open access repository and the other to the published version, access to which will be dependent on a subscription:
The following are specific open access search engines:
Gives you access to millions of scholarly articles aggregated from over 600 Open Access repositories.
Access to millions of scholarly articles and other materials (e.g. theses) from more than 3,000 sources
Open access to over 900,000 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics
SocArXiv, open archive of the social sciences, provides a free, non-profit, open access platform for social scientists to upload working papers, preprints, and published papers, with the option to link data and code.
bioRxiv is a free online archive and distribution service for unpublished preprints in the life sciences. It is operated by Cold Spring Harbor Laboratory, a not-for-profit research and educational institution.
The OSF is a free, open source service of the Center for Open Science, and provides a platform for many disciplinary preprint services including AgriXiv, PsyArXiv and SportRxiv.