➔ The Mason OER Metafinder launches a real-time, simultaneous search across 22* different sources of open educational materials.
➔ Another very good index is OASIS. Developed at SUNY Geneseo's Milne Library and built by Bill Jones, Digital and System Resources librarian and Ben Rawlins, Library Director at SUNY Geneseo and colleagues working on their own time.
Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 87 different sources and contains 339,302 records.
Not surprisingly, you can use Google to search for OER. Type in a subject term and find your way to Advanced Search. The location of Advanced Search tends to change; currently (Fall 2022) it can be found by clicking the Settings gear wheel icon and scrolling down.
Once there, scroll to Usage Rights near the bottom of the page, and choose the level of access you prefer.