Open Access books and journals are "digital, online, free of charge, and free of most copyright and licensing restrictions (1)."
You can link to OA works as free course readings, but there may be other limits on their use that differ from OER. For example, an Open Access book may not allow retaining (i.e. downloading) or remixing.
(1) For more information about Open Access, see Peter Suber's Open Access Overview.
OASIS (Openly Available Sources Integrated Search) tool makes the discovery of open content easier, including content from over 90 sources. Good balance of textbooks and other content (lesson plans, courses, videos, etc).
