Licensing is key when you're looking at incorporating existing resources into your OER. If you want to adapt an existing resources, you'll want to make sure that it's licensed to allow modifications (OR that you get the author's permission to adapt their material).
Several Creative Commons licenses allow remixing. Look for these when you're collecting materials for your project (especially if you know that you want to adapt existing content): CC Attribution (CC BY), CC Attribution Non-Commercial (CC BY NC), CC Attribution Share-Alike (CC BY SA), CC Attribution Non-Commercial Share-Alike (CC BY NC SA). See the chart below for summaries of these licenses.
When you're remixing OER, you will want to know how to properly cite other people's work. All Creative Commons licenses come with the expectation of attribution, even if you are adapting content. Check out this attribution builder from Open Washington and this explanation of attribution statements by BCcampus. In your citation, be sure to include:
Here's a sample citation for an adapted work: