"Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions.
What makes it possible is the internet and the consent of the author or copyright-holder."
From Peter Suber's Very brief introduction to Open Access.
Learn more : Open Access and Scholarly Publishing
These databases allow you to search specifically for OA dissertations and theses:
Boston College is a member of HathiTrust, a repository of millions of full text books and other materials. Download them by authenticating with your BC username and password.
Search HathiTrust content by keyword:
Search these catalogs for open access books and other media: