Have you found Open Educational Resources you want to reuse? Then the first step is to check what exactly you are allowed to do with the material in question. A material’s licence indicates what is allowed. With regard to the use of Open Educational Resources, reference is often made to David Wiley's 5 Rs:
As the 5R Model shows, the manner in which materials may be reused can vary. Therefore, before reusing a specific material, always check its licence, which is usually attached to it. If you do not find a licence, the material cannot be reused without permission. If the material is accessible online, linking is allowed.