There are hundreds--if not thousands--of websites that provide OERs. Finding and searching all of these websites would be overwhelming.
The resources in the OER | Broad Search tab search multiple OER websites: These resources are called federated search engines.
Note that federated searches sometimes take a bit longer than pre-indexed curated content. If that is the case, just be patient--since in the long run you'll be saving yourself time.
Resources listed in the other tabs below are in no way are all that's available; they are this librarian's top picks, with a focus on federated search websites or large repositories.