As previously noted, OER are basically anything you can utilize in a course that are open to use - from full courses to textbooks, from individual modules to media components. Unfortunately, there isn't a single tool that harvests everything for us, so below are some resources that you can use to help identify OER materials.
You might also want to try the search tools below.
George Mason University's OER Search Tool:
SUNY Geneseo, Milne Lbrary's OER Search Tool: