These are some guides from a variety of institutions who have organized OER by subject or discipline.
Washtenaw Community College - Subject Disciplines
Tidewater Community College - OER by Discipline
Lansing Community College - OER by Discipline
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 79 different sources and contains 324,517 records.
OASIS is being developed at SUNY Geneseo's Milne Library.
The BCcampus Open Education OER by Discipline Guide lists a wide range of open educational resources organized by discipline. This guide is updated as new resources are identified. Note that textbooks in the BC Open Textbook Collection are not included in this guide.
Mason OER Metafinder (MOM)The OER Metafinder performs a simultaneous search across 19 different sources of open educational materials. From George Mason University.