"Open Access is the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment."(SPARC, the Scholarly Publishing and Academic Resources Coalition).
There are multiple ways for scholarly works to be open access:
The repositories can be at institutional or central level, or can be specialized in a specific subject.
These repositories are maintained by major funders.
Discovering each institutional repository is a difficult task. Instead, it is recommended the use of specialized search engines to discover repositories of interest.
Check this list for more disciplinary repositories:
Open access journals are journals freely available to readers. These journals are published by open access publishers or by traditional publishers.
Find open access journals titles from open access publishers:
Publishers can offer a mix of traditional and open access articles or they offer free delayed access to their publications. See the individual journal home pages and check what articles are available for free.