Census of Institutional Repositories in the United States
MIRACLE Project Research Findings
gives some background to where OA material is being stored.
The report is 167 pages but their summary gives a sense:
In this report, the authors describe results of a nationwide census of institutional repositories in U.S. academic institutions. The census is one of several activities of the MIRACLE Project, an IMLS-funded research program based at the University of Michigan.
A considerable portion of the scholarly record is born digital, and some scholarship is produced in digital formats that have no physical, in-the-hand counterparts. The proliferation of digital scholarship raises serious and pressing issues about how to organize, access, and preserve it in perpetuity. The response of academic institutions has been to build and deploy institutional repositories (IRs) to manage the digital scholarship their learning communities produce.Richard Baer