Open Access (OA) materials are freely available but often do not allow retaining, redistributing, remixing, or revising. Look for a Creative Commons License before assuming freely available material is OER.
Search within these excellent collections of OER materials to find even more options. The best places to search may vary for your field. Please don't hesitate to ask a librarian for suggestions!
Search in Spartan Search, the library catalog, to find thousands of open-access books and articles, some of which are OER.
Use the "open access" filter on the left side of your results to narrow to this type of material. As a reminder, OA materials are freely available but usually do not allow retaining, redistributing, remixing, or revising as an OER would.