Do pay careful attention to the licenses (typically, Creative Commons, but not always) and permissions that have been assigned to open educational resources (OER). That information will guide your usage of those works. Specifically, consider whether you will you be able to retain, revise, remix, reuse and redistribute (Wiley 2020) as needed.
This page profiles the main features of the Creative Commons licensing system.
If you are in need of assistance, do Ask a Librarian.