Assigning a Creative Commons License to OER provides a simple, standardized way to give others permission to share and use your work -- on conditions of your choice. You retain copyright of your work while allowing others to make limited uses.
There are six types of Creative Commons License (See comparison of the licenses in the box below).
Assigning a CC-BY license to your educational resource would give others most freedom to use, adapt, re-mix, and redistribute them while giving you credits.
Non-derivative license does not permit others to modify your work, thus making it not an OER.
As the content creator, one can relinquish your copyright claim on the creation. Creative Commons has a Public Domain or CC0 license that marks a work as completely open with no restriction or requirement attached to any type of reuse.
See Guide to Using Public Domain Tools from Creative Commons.