Most search engines, databases, repositories, archives, and other places where information is gathered will have an 'Advanced Search' feature. How this looks will vary from site to site, but there are several common features shared among them.
For OER materials, you will need to look for something called 'licenses', 'usage rights', or 'creative commons' as this will let you filter by whether or not you are allowed to use the item in question freely or not.
Below are some helpful tips on how to find these boxes on specific, commonly used websites.
It's recommended that you narrow your search by language when using advanced search! Otherwise you may end up with great resources in a language you cannot read. You can find the language selection under "Then narrow your results by" section of advanced search.
The Mason OER Metafinder is a real-time search engine that searches through several OER websites and databases. It can be a great resource, but can be a little more intimidating as it's less like Google and more like a full database!