There are several compilations of OER organized by discipline and field. The difficulty is that they must be regularly updated because new OER are being created and shared every day.
The OER by Discipline Directory is one of the most trustworthy. It is maintained by BCcampus at the University of British Columbia, one of the major publishers of open textbooks in North America.
On the subpages of this section you will find OER initiatives and disciplinary collections organized by division - Humanities, Natural Sciences, Social Sciences - and school - Nursing, Social Work, and so on. More schools and divisions will be added soon.