Depends. If you look at OMM as a process of health care distinct from "allopathic medicine" then the evidence for the effectiveness of OMM is rather weak. If you look at spinal manipulation as a treatment modality, then the picture is mixed. Here is the latest pragmatic effectiveness studies of spinal manipulation combined with exercise for back pain...
AMA News Article Comparing MD and DO styles
BMJ. 2004 Nov 29
United Kingdom back pain exercise and manipulation (UK BEAM) randomised trial: effectiveness of physical treatments for back pain in primary care.
[No authors listed]
OBJECTIVE: To estimate the effect of adding exercise classes, spinal manipulation delivered in NHS or private premises, or manipulation followed by exercise to "best care" in general practice for patients consulting with back pain. DESIGN: Pragmatic randomised trial with factorial design. SETTING: 181 general practices in Medical Research Council General Practice Research Framework; 63 community settings around 14 centres across the United Kingdom. PARTICIPANTS: 1334 patients consulting their general practices about low back pain. MAIN OUTCOME MEASURES: Scores on the Roland Morris disability questionnaire at three and 12 months, adjusted for centre and baseline scores. RESULTS: All groups improved over time. Exercise improved mean disability questionnaire scores at three months by 1.4 (95% confidence interval 0.6 to 2.1) more than "best care." For manipulation the additional improvement was 1.6 (0.8 to 2.3) at three months and 1.0 (0.2 to 1.8) at 12 months. For manipulation followed by exercise the additional improvement was 1.9 (1.2 to 2.6) at three months and 1.3 (0.5 to 2.1) at 12 months. No significant differences in outcome occurred between manipulation in NHS premises and in private premises. No serious adverse events occurred. CONCLUSIONS: Relative to "best care" in general practice, manipulation followed by exercise achieved a moderate benefit at three months and a small benefit at 12 months; spinal manipulation achieved a small to moderate benefit at three months and a small benefit at 12 months; and exercise achieved a small benefit at three months but not 12 months
BMJ. 2004 Nov 29
United Kingdom back pain exercise and manipulation (UK BEAM) randomised trial: cost effectiveness of physical treatments for back pain in primary care.
OBJECTIVE: To assess the cost effectiveness of adding spinal manipulation, exercise classes, or manipulation followed by exercise ("combined treatment") to "best care" in general practice for patients consulting with low back pain. DESIGN: Stochastic cost utility analysis alongside pragmatic randomised trial with factorial design. SETTING: 181 general practices and 63 community settings for physical treatments around 14 centres across the United Kingdom. PARTICIPANTS: 1287 (96%) of 1334 trial participants. MAIN OUTCOME MEASURES: Healthcare costs, quality adjusted life years (QALYs), and cost per QALY over 12 months. RESULTS: Over one year, mean treatment costs relative to "best care" were pound195 ($360; 279 euro; 95% credibility interval pound85 to pound308) for manipulation, pound140 ( pound3 to pound278) for exercise, and pound125 ( pound21 to pound228) for combined treatment. All three active treatments increased participants' average QALYs compared with best care alone. Each extra QALY that combined treatment yielded relative to best care cost pound3800; in economic terms it had an "incremental cost effectiveness ratio" of pound3800. Manipulation alone had a ratio of pound8700 relative to combined treatment. If the NHS was prepared to pay at least pound10 000 for each extra QALY (lower than previous recommendations in the United Kingdom), manipulation alone would probably be the best strategy. If manipulation was not available, exercise would have an incremental cost effectiveness ratio of pound8300 relative to best care. CONCLUSIONS: Spinal manipulation is a cost effective addition to "best care" for back pain in general practice. Manipulation alone probably gives better value for money than manipulation followed by exercise.