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.
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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
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For more information, visit Best practices for attribution (Creative Commons).
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For a detailed legal overview of the Creative Commons licenses, see Creative Commons Licenses Legal Pitfalls: Incompatibilities and Solutions.