Open educational resources (OER) are freely accessible, openly licensed text, media, and other digital assets that are useful for teaching, learning, and assessing as well as for research purposes. There is no universal usage of open file formats in OER. The term OER describes publicly accessible materials and resources for any user to use, re-mix, improve and redistribute under some licenses. 
The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is either (1) in the public domain or (2) licensed in a manner that provides everyone with free and perpetual permission to engage in the 5R activities :
5Rs image by David Wiley, Lumen Learning, licensed under CC BY 4.0
 Bell, Steven. "Research Guides: Discovering Open Educational Resources (OER): Home". guides.temple.edu. Retrieved 2020-02-11.
OER Commons - OER Commons is a public digital library of open educational resources. Explore, create, and collaborate with educators around the world to improve curriculum.
MERLOT - Created by the California State University System. A collection of 45k+ resources that are rated, peer-reviewed, and tallied by how many "personal collections" each resides in. Lists more than 3k open textbooks, some of which have accessibility information for students with disabilities.
MIT OpenCourseWare Online Textbooks - Index to open textbooks provided by MIT covering a wide variety of subjects.
Noba - OER textbook resources for psychology.
Open Attribution Builder - This is a tool to help you build attributions.
BCcampus Open Education Self-Publishing Guide - The BCcampus Open Education Self-Publishing Guide is a reference for individuals or groups wanting to write and self-publish an open textbook. This guide provides details on the preparation, planning, writing, publication, and maintenance of an open textbook. Copyright, open-copyright licenses, and the differences between citation and attribution are discussed as well as the importance of copy editing and proofreading. Checklists and templates are also provided.
Open Library - Open Library is an open, editable library catalog, building towards a web page for every book ever published. Over 25,000 books listed under the Medicine subject heading.