Open Educational Resources, or "OER" are defined as "teaching, learning, and research materials that are either (a) in the public domain or (b) licensed in a manner that provides everyone with free and perpetual permission to engage in the "5R activities".
The 5R activities are:
(from the Creative Commons website)
OER textbooks are not necessarily in the public domain, but are freely available to download and use following the Creative Commons Licensing (usually just cite!)
Most of the time their license allows educators to share and remix content, but check each text individually to be sure.