Dr. Ostrom is the director of Mount Mercy Hospital for the Criminally Insane. He was the attending physician for Cornelia Delamere Pendergast and Constance Greene during their time at the facility.

Ostrom is tall and fastidious, often dressed in an expensive suit beneath a white medical jacket. His manner, while cordial and pleasant with patients, could best be described as "reluctantly accommodating" to visitors.

An unknown past incident during one of Aloysius Pendergast's visits to his Aunt Cornelia caused Ostrom to be particularly circumspect with her visitors, whether Pendergast himself or Vincent D'Agosta, who insisted on posing as Cornelia's long-dead brother Ambergris during his visit.

A few years later, Ostrom encountered Pendergast again when the agent arranged for Constance Greene to be transferred to Mount Mercy following Cornelia's sudden death. When Judson Esterhazy arrived posing as one of Constance's former doctors, Ostrom—at the urging of Constance's court-appointed psychiatrist, Dr. John Felder—hesitantly agreed to an off-site outing, during which Constance was kidnapped by Esterhazy. Though she was returned to the facility unharmed thanks to Pendergast and D'Agosta, Ostrom and Felder each endured a severe dressing-down by Pendergast prior to Constance's rescue. Afterward, Ostrom severely restricted Felder's access to Constance, imposing several rules on the visits and making his distaste for Felder's presence quite clear.
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