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Importance: There are 2 degree programs for licensed physicians in the US: allopathic medical doctorate (MD) and osteopathic doctorate (DO). However, evidence is limited as to whether outcomes differ between patients treated by MD vs DO surgeons., Objective: To evaluate differences in surgical outcomes and practice patterns by surgeon medical school training (MD vs DO)., Design, Setting, and Participants: This retrospective cohort study used 100% Medicare claims data from inpatient hospitals providing surgical services from January 1, 2016, to December 31, 2019 among Medicare fee-for-service beneficiaries aged 65 to 99 years who underwent 1 of the 14 most common surgical procedures. Data analysis was performed from January 17, 2023, to August 13, 2024., Exposure: Medical school degree (MD vs DO)., Main Outcomes and Measures: The primary outcome was 30-day mortality, and the secondary outcomes were readmissions and length of stay. To assess differences between surgeons by medical school training, a multivariable linear probability model was used, which was adjusted for hospital fixed effects and patient, procedure, and surgeon characteristics., Results: Of the 2 360 108 total surgical procedures analyzed, 2 154 562 (91.3%) were performed by MD surgeons, and 205 546 (8.7%) were performed by DO surgeons. Of 43 651 total surgeons, most surgeons were MDs (39 339 [90.1%]), the median (SD) age was 49.0 (9.8) years, and 6649 surgeons (15.2%) were female. The mean (SD) age of patients undergoing surgical procedures was 74.9 (6.7) years, 1 353 818 of 2 360 108 patients (57.4%) were female, and 2 110 611 patients (89.4%) self-reported as White. DO surgeons were significantly more likely to operate on older patients (DO patient mean [SD] age: 75.3 [7.1] years; MD patient mean [SD] age: 74.8 [6.6]), female patients (DO: 60.2% of patients; MD: 57.1% of patients), and Medicaid dual-eligible patients (DO: 10.3% of patients; MD: 8.6% of patients). DOs performed a lower proportion of elective operations (DO: 74.2% vs MD: 80.2%) and were more likely to work in public hospitals (DO: 10.3%; MD: 9.5%) and nonteaching hospitals (DO: 57.9%; MD: 68.1%). There was no evidence that 30-day mortality differed between MD and DO surgeons (adjusted mortality rate, DO: 1.61%; MD: 1.58%; absolute risk difference [aRD], -0.04 percentage points; 95% CI, -0.11 to 0.04; P = .37). On secondary analyses, no difference was found in 30-day readmissions or length of stay between MD and DO surgeons., Conclusions and Relevance: In this retrospective cohort study using Medicare data, there was no evidence that patient outcomes differed between MD and DO surgeons for common operations after adjusting for patient factors and practice settings.