Although obstetrician-gynecologists (OB-GYNs) are the main actors in the provision of health care to women, their practice patterns have rarely been analyzed. The current study investigated the nationwide ambulatory visits to OB-GYNs in Taiwan using the National Health Insurance Research Database. From the 1/500 sampling datasets indicating 619,760 ambulatory visits in 2012, it was found that 5.8% (n = 35,697) of the visits were made to OB-GYNs. Two-fifths of the services provided were performed by male OB-GYNs aged 50–59 years. Women of childbearing age accounted for more than half of the visits to OB-GYNs (57.2%), and elderly patients above 60 years accounted for only 7.7%. The most frequent diagnoses were menstrual disorders and other forms of abnormal bleeding from the female genital tract (13.1%). Anti-infective agents were prescribed in 15.1% of the visits to OB-GYNs. The study revealed the proportion of aging practicing OB-GYNs, and our detailed results could contribute to evidence-based discussions on health policymaking.