Open Educational Resources are materials that can be used to supplement instruction with little or no cost, and are typically licensed for re-use or are in the public domain (meaning you don't have to ask for permission to use them in your classes). They can include presentations, activities, quizzes, textbooks, course modules, and more.
This guide is a collection of OER organized by discipline. If yours isn't listed, check out the Multi-Disciplinary page.
The Creative Commons is designed to facilitate the sharing of materials by offering mix-and-match licensing. Creators of content can choose to explicitly license their works with their preference of attribution, derivatives, commercial works, and further sharing, or they can use a CC0 license and put work into the public domain. These works are your best bet for recently-created materials that are re-usable and modifiable.
The Creative Commons licensing structure is simply meant to be an easy, pre-packaged bundle of licenses for creators to expressly allow re-use of the works, but open licensing doesn't necessarily have to take the form of CC. Even supplemental materials provided by textbook publishers may be licensed for re-use or modification.
Materials in the public domain are those that aren't under a current copyright, whether that's because they were created prior to 1923, their copyright wasn't renewed during the 20th century, or they were deliberately released into the public domain (CC0 license). You can do whatever like you with these materials: re-use, remix, re-distribute! There are no licensing or fair use considerations.