When using OER materials make certain you understand the license associated with the text. Most OER texts will have Creative Commons licensing. If a text is licensed with a no derivatives 
Creative Commons license you cannot remix it to create a new OER text.
You may be able to include works from an OER text under the fair use doctrine. In most cases, linking to an outside work will be the safest way to use the material. If you plan to use the text for an extended period of time, fair use is likely not going to be in play for an OER text.
Public domain works exist outside copyright restrictions. Often the intellectual rights have expired. Public domain works can be used in OER texts, but be careful. Translations and other derivative works may still be subject to copyright restrictions even if the original work is in the public domain.