You may have already discovered online websites that provide access to resources, some of which are open, but you want a way to filter out the proprietary resources and leave only those resources that are free-to-use OER.
Some sites, like Wikipedia, offer everything under one Creative Commons licence. In Wikipedia’s case, all text is licensed under a CC BY SA licence. OpenClipart offers everything under a CC Zero licence, which is basically Public Domain and allows users to take and use the artwork as they need.
Many resource-sharing websites provide the opportunity to filter search results according to various CC licenses, such as Flickr (photographs), YouTube (video) and SlideShare (presentations) and Google Advanced Search (web resources both text and images).
The following video provides some guidance as to how these filters work. Be aware that on sites such as these, you need to check the licence conditions to reuse the resource, even though the resource may appear to be freely available.