Open Educational Resources are not limited to textbooks, they can be course materials, modules, images, videos, learning objects, tests, quizzes, lessons plans, syllabi, simulations, and any other materials that serves as educational and conforms with the definition of OER.
The 5 activities or permissions proposed by David Wiley are: Reusing, Redistributing, Retaining, Remixing and Revising.
All open licenses provide at least these 2 permissions: Reusing and Redistributing.
Some open licenses also permit Remixing or combining OER with other materials to create new OER. You may also Revise, or edit the content to fit the learning objectives. And you are also allowed to Retain or archive your own copies of the work.