Open Educational Resources (OER) are teaching and learning materials that you may freely use and reuse at no cost, and without needing to ask permission. Unlike copyrighted resources, OER have been authored or created by an individual or organization that chooses to retain few, if any, ownership rights. OER may include full course curricula, course materials, modules, textbooks, media, assessments, software, and any other tools, materials, or techniques, whether digital or otherwise, used to support access to knowledge.
In some cases, that means you can download a resource and share it with colleagues and students. In other cases, you may be able to download a resource, edit it in some way, and then re-post it as a remixed work. How do you know your options? OER often have a Creative Commons license or other permission to let you know how the material may be used, reused, adapted, and shared. (From OER Commons)