In order to fully understand OER, you need to understand a little bit about copyright.
"Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device" (17 U.S.C. § 102).
Copyright refers to a creator's exclusive right to duplicate, distribute, tweak, remix, or publicly perform or display their works. Copyright protects fixed, original, and creative works. Further, copyright protection exists whether or not there is a notice included with the work--it is presumed the moment a work is created and fixed in a tangible form.
Copyright Basics
There are many excellent resources that give an overview of the essentials of copyright.