The key distinguishing characteristic of OER is its intellectual property license and the freedoms the license grants to others to share and adapt it. If a lesson plan or activity is not clearly tagged or marked as being in the public domain or having an open license, it is not OER. The most common way to release materials as OER is through Creative Commons copyright licenses (pictured below).
You can learn more about Creative Commons Licenses here: https://creativecommons.org/