Coursepacks are materials pulled together from various sources that either supplement or replace the textbook. Since they can often be printed and sold at low cost, they are usually more affordable than traditional textbooks. But that doesn't necessarily mean that they are open educational resources, because some or all of the content may not have an open license.

If you plan to compile or remix a variety of openly licensed or public domain materials into a coursepack, then your work is an OER. You can put an open license on it and share widely. 

If you plan to create a coursepack of materials that are under copyright, talk to a librarian. They can help you figure out whether you have a good fair use argument if you are challenged. They can also help you find content that is already licensed for use through the library. This kind of coursepack should not be shared widely, because you don't have the right to redistribute other people's copyrighted work.

When you change your course materials, follow your institution or department's procedures to make the change official. The bookstore and other departments need to know so that they can stock the right materials and let students know about no-cost and low-cost course materials that are in use.