What makes OER truly "open" are the permissions to Revise, Remix, Reuse, Retain, and Redistribute the content contained in them. These 5R permissions are what make OER different from material which is copyrighted under traditional, all-rights-reserved copyright.
The creator/licensor retains copyright while granting permissions.
Another way to frame this is that OER can accurately be described as:
open = free + permissions (the 5Rs)
These permissions help users of openly licensed content understand what they are allowed to do with the work. These permissions are granted in advance and are legally established through Public Domain or Creative Commons licensing:
Revise – the right to adapt, adjust, modify, or alter the content itself (e.g., translate the content into another language)
Remix – the right to combine the original or revised content with other material to create something new (e.g., incorporate the content into a mashup)
Reuse – the right to use the content in a wide range of ways (e.g., in a class, in a study group, on a website, in a video)
Retain – the right to make, own, and control copies of the content (e.g., download, duplicate, store, and manage)
Redistribute – the right to share copies of the original content, your revisions, or your remixes with others (e.g., give a copy of the content to a friend)