While Creative Commons licensing is a well-recognized, straight-forward, flexible type of licensing that is strongly associated with OER, there are other types of open licensing available.
The GNU General Public License is a “free, copyleft license for software and other kinds of works.”
Works in the public domain are not copyrighted. Different countries have different copyright laws and lengths, however, which must be carefully checked.
Creative Commons has tools that allow users to place their works in the public domain.