Whether you are re-mixing other OER to create your own OER or just assigning OER for a class, you should properly credit the original OER. All materials released under Creative Commons (CC) licenses require attribution. If something is labeled CC0, it is public domain, not under a CC license, and does not require attribution.
All CC attributions should include the following basic information:
For example, this guide should be cited as follows: "Open Educational Resources (OER)" by Salisbury University Libraries, licensed under CC BY 2.0
For more information, visit Best practices for attribution (Creative Commons).
For a detailed legal overview of the Creative Commons licenses, see Creative Commons Licenses Legal Pitfalls: Incompatibilities and Solutions.