Dr. J.A.O. Brennan was born in 1880. His parents named him after Dr. J.A. Ochterlony, a prominent Louisville physician. He graduated from the University of Louisville School of Medicine in 1901 under Dr. Ochterlony. Afterward, Dr. Brennan spent three years in Europe, primarily in London, Vienna, Paris and Berlin for post-graduate study. Upon his return to Louisville, Dr. Brennan worked at St. Mary's Hospital, St. Elizabeth's Hospital, and St. Anthony's Hospital, all in Louisville.
In 1912, Dr. Brennan added the doctor's office and waiting and exam rooms to the north side of the Brennan House. He was a surgeon and general practitioner in this office until shortly before his death in 1963 at the age of 83. He never owned an automobile, so he walked or rode the trolley around town. In emergencies, he took a carriage and later a taxi to his destinations. At one time, he rented offices in the Weissinger-Gaulbert building on 3rd Street.
Architect C.A. Curtin drew a blueprint for Dr. Brennan showing the potential addition to the Brennan House. Curtin replicated the original architecture of the house to ensure that the office addition did not damage the aesthetics of the home. The bay window in Thomas Brennan’s study was bricked in and a doorway to the doctor’s office was added in the center window.