In a major research report to be published in the December 2015 issue of Journal of Computing in Higher Education it was concluded that ” In three key measures of student success—course completion, final grade of C- or higher, course grade– students whose faculty chose OER generally performed as well or better than students whose faculty assigned commercial textbooks” (Fischer et al 2015). You can access the article from the above link. The citation is below. Here is a link to a Campus Technology Oer article discussing the findings.
Fischer, L., Hilton, J., Robinson, J., Wiley, D. (2015). A multi-institutional study of the impact of open textbook adoption on the learning outcomes of post-secondary students, Journal of Computing in Higher Education, December 2015, Volume 27, Issue 3, pp 159-172