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Open Educational Resources (OER) are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and repurposing by others. OER include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge. (William and Flora Hewlett Foundation)
OER is generally thought to be online-only material and courses. However, many open textbooks, for example, are also available in hard copy, or can be printed if preferred. ( "Faculty OER Toolkit" by Shannon Moist)