長岡市
Ohara was born in what is now Nagaoka, Niigata as the third son of Tanaka Keijiro, a former samurai, the son of an impoverished former samurai, but was later adopted by Ohara Tomotada, a former samurai from Aizu Domain, and took the Ohara surname. One of his early classmates was the future diplomat Matsudaira Tsuneo. He graduated from the law school of Tokyo Imperial University in July 1902 and received a posting to the Ministry of Justice (Japan) as a public prosecutor. He served in that capacity for the next few years with the Shizuoka District Court, the Tokyo District Court and the Chiba District Court, and made a name in several trials involving high-profile corruption scandals, notably the Ōura scandal and Siemens scandal, as well as in the High Treason Incident, where he was the lead interrogator for prosecuting Kanno Sugako.