Creative Commons provides creators with a way to manage the copyright permissions of their works, usually with the specific purpose of allowing the reuse and sharing of their material. Having a Creative Commons license attached to a work signals to others that specific permissions are granted. CC licenses may be applied to any type of work, including educational resources, music, photographs, databases, government and public sector information, and many other types of material.
For information about licensing a work using Creative Commons, check out this page: https://creativecommons.org/share-your-work/
For details about Creative Commons and OER: https://creativecommons.org/category/education-oer/
There are plenty of tools online to help you find Creative Commons resources.
Creative Commons offers a "CC Search" tool to assist you with identifying resources that have a Creative Commons license.
Google and Bing both have filters when searching for images in their advanced search options that let you limit results to Creative Commons resources.
YouTube also has a filter for Creative Commons videos.