Since at least 1904, local accounts have confused Ōyama Iwao with Sukeichi Oyama (1858-1922), Japanese engraver who studied at Temple Hill Academy in Geneseo, New York, United States.

[3][4] In 1870, Ōyama was sent overseas to the École spéciale militaire de Saint-Cyr in France (August 1870 – March 1871)[2] to study and was appointed the official Japanese military observer to the Franco-Prussian War.

[5] On his return home, he helped establish the fledgling Imperial Japanese Army, which was soon employed in suppressing the Satsuma Rebellion,[2] although Ōyama and his elder brother were cousins of Saigō Takamori.

In the First Sino-Japanese War, Ōyama was appointed the commander-in-chief of the Japanese Second Army,[2] which after landing on Liaodong Peninsula, carried Port Arthur by storm, and subsequently crossed to Shandong, where it captured the fortress of Weihaiwei.

He was replaced by General Kodama Gentarō briefly during early 1905 due to illness, but recovered to direct Japanese forces in the final Battle of Mukden.

As the War Minister in several cabinets and as the Chief of the Army General Staff, Ōyama upheld the autocratic power of the oligarchs (genrō) against democratic encroachments.

From 1914 to his death he served as the Lord Keeper of the Privy Seal (内大臣, naidaijin),[2] and in this capacity attended the accession ceremony of the Emperor Taishō, which took place in Kyoto in November 1915.

[13] The house was destroyed by the Great Kantō earthquake in 1923 or possibly by American air raids during World War II [citation needed].