For most people understanding U.S. Copyright law is challenging and often frustrating. The complexities of understanding your rights as a copyright owner, as a user of third-party works, even as an educator, especially in a digital environment take time and effort. Laws pertaining to copyright change over time, and the purpose of copyright "to promote the progress of science and the useful arts" (I U.S. Constitution S8, c8) sometimes seems ambiguous, especially for those who are not legal experts.
While extremely specific use exemptions exist for classroom teaching, libraries, and online learning (see "Copyright in Teaching"), fair use and obtaining permission (sometimes paying a fee) are also options, the advent of authors applying Creative Commons licenses to their works has greatly broadened the availability of creative, original works which may be used with attribution as designated by their authors.
Creative Commons Licensing alleviates the ambiguity of use for many online resources. They are free for anyone to use and allow for resources to be adapted and shared universally. There are six licenses, four of which are considered "open" with varying degrees of permissions. OER are published using one of the four licenses and adhere to the 5Rs: revise, remix, reuse, redistribute, and retain, allowing you to edit and reorganize a text, combine multiple OER, recycle and update the same resource year after year, share any alterations you make (typically under a similar license), and keep a copy without fear of it disappearing (as can be the case with licensed ebooks or journal subscriptions).