Creative Commons and other open licenses require you to attribute the CC-licensed materials you use. Here's how.
By definition, OER's are materials that have either been marked with an open license, such as Creative Commons, or that have fallen into the public domain.
Works in the public domain are not protected by copyright, so you can re-use them freely without permission, because:
Copyright has expired
The work was never protected by copyright
The work was donated to the public domain
It's a US government publication
Creative Commons is the most widely used type of open license. CC licenses are free, standardized licenses that you can use to share your creations, and to define the ways in which others may use those creations.