In terms of studies about OMM, these may be useful:
Study: American Osteopathic Association guidelines for osteopathic manipulative treatment (OMT) for patients with low back pain. (2010). The Journal Of The American Osteopathic Association, 110(11), 653-666.
Conclusion: Osteopathic manipulative treatment significantly reduces low back pain. The level of pain reduction is clinically important, greater than expected from placebo effects alone, and may persist through the first year of treatment. Additional research is warranted to elucidate mechanistically how OMT exerts its effects, to determine if OMT benefits extend beyond the first year of treatment, and to assess the cost-effectiveness of OMT as a complementary treatment for low back pain.
Study: Jäkel, A., & von Hauenschild, P. (2011). Therapeutic effects of cranial osteopathic manipulative medicine: a systematic review. The Journal Of The American Osteopathic Association, 111(12), 685-693.
Conclusion:The currently available evidence on the clinical efficacy of cranial OMM is heterogeneous and insufficient to draw definitive conclusions. Because of the moderate methodological quality of the studies and scarcity of available data, further research into this area is needed.
Study: Draper, B., Johnson, J., Fossum, C., & Chamberlain, N. (2011). Osteopathic medical students' beliefs about osteopathic manipulative treatment at 4 colleges of osteopathic medicine. The Journal Of The American Osteopathic Association, 111(11), 615-630.
Conclusion: The reason why a student decided to study osteopathic medicine was strongly associated with the level of agreement with osteopathic philosophy and the intention to use OMT in future practice. Prior experience receiving OMT, the medical school that a student attends, and the current year of study appear to be related to the students' levels of agreement with osteopathic philosophy and intention to use OMT.
Those are just some of the studies out there on OMM/OMT. Most of the literature says that further research is needed (duh).