There is a more detailed statement about copyright and fair use at this page.

Each book has copyright and fair use information on its title page. Most of the tiles on the OLL were published before 1923 and so are in the public domain. For example, John Locke's Two Treatises of Civil Government (1764 edition) has the following copyright statement: "The text is in the public domain." And the following fair use statement: "This material is put online to further the educational goals of Liberty Fund, Inc. Unless otherwise stated in the Copyright Information section above, this material may be used freely for educational and academic purposes. It may not be used in any way for profit."

Books published by Liberty Fund to which it has electronic rights have slightly more restrictions on their use. To make copies and distribute them for other purposes (such as college course booklets) requires explicit permission from Liberty Fund, which is usually given. 

There are a number of other books which are published online under license (such as the Glasgow Edition of the Works of Adam Smith, the Works and Correspondence of David Ricardo, and the Collected Works of J.S. Mill). To copy or distribute these works requires the express permission of the copyright holders, namely Oxford University Press, Cambridge University Press, and the University of Toronto Press, respectively). Please consult the relevant title pages of these works for details.