When a work is in the public domain, it is free for use by anyone for any purpose without restriction under copyright law. As such it is similar to the concept of OER and has been used to describe non-copyrighted materials free for use by public. Public domain is considered the purest form of open/free, since no one owns or controls the material in any way.
Works that are in the public domain in one legal jurisdiction are not necessarily in the public domain worldwide. Copyright laws differ from jurisdiction to jurisdiction, both in the duration of protection and what constitutes copyrightable subject matter. For example a US Government work clearly in the public domain in the United States may or may not be free of copyright restrictions and in the public domain in other jurisdiction. At present, one of the only ways to be certain that a particular work is in the public domain worldwide is to see if the copyright holder has dedicated all rights to the work to the public domain by using CC0, which is the “no copyright reserved” option in the Creative Commons toolkit. CC0 is a single purpose tool, designed to take on the dedication function of the former, deprecated Public Domain Dedication and Certification (Source).
Public Domain, PD, Creative Commons, CC, CC0, OER