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OERs are issued under a range of different licences. You should always consult the licence of the OER you wish to use. There may be limits to how you can adapt or build upon the resource, or you may have to follow specific rules regarding attribution.
Many (but not all) OERs are issued under a Creative Commons Licence.
The Creative Commons website contains lots of information about how you can use material that has been issued under a CC license.
Consider adding open textbooks to your Reading Lists. Open textbooks don't generally have limits on their access, so you can be confident that any number of students can access the resource at the same time. Contact your Academic Librarian to edit your reading list and make more use of open resources.