Unless you have contractually transferred your copyright to an academic publisher by contract, you are the copyright owner of your work and therefore have the right to make it Open Access. When an article is published in a journal or book, authors often sign a copyright transfer agreement, and the publisher's terms and conditions apply. Authors who wish to retain at least some of their rights can use the SPARC Author Addendum. Of course, the publisher must agree to any addendum.
Most toll-access publishers allow self-archiving (Green OA), sometimes after an embargo period.
You can check the policy of each journal on the SHERPA/RoMEO website. Open Access Button has created a very handy tool, Shareyourpaper.org, which allows authors to automatically and easily check how they can legally share their work.
Open Access Button has also developed another tool, called Direct2AAM, to provide authors with instructions on how to find their Author Accepted Manuscript (postprint) in the journal's submission system.
Scholarship@Claremont (S@C) archives research produced by The Claremont Colleges faculty and students. It contains records of all books, book chapters, journal articles and working papers written by the professors and researchers. Records of journal articles or papers may be accompanied by a full-text PDF file, to which access may be open or restricted, depending on the statute of the file. If necessary, an embargo period can be set. The repository is registered in OpenDOAR, and allows the researchers of The Claremont Colleges to meet the Open Access requirements of their funders.