The Mason OER Metafinder allows you to simultaneously search across the multiple OER repositories (e.g., MERLOT II, OAOpen.org, OER Commons, OpenStax CNX, etc.) as well as many open archival repositories (e.g. HathiTrust, Internet Archive, Project Gutenberg, etc.).
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 52 different sources and contains 155,375 records. Great place to search when you know what type (audio, textbook, etc.) of material you are looking for.
These OER repositories include textbooks, course materials, videos and more. As you search, look for ways to narrow your search to Higher Ed resources.