Oyama was born in Kagoshima to a samurai family of the Satsuma Domain. He was a nephew of Saigo, with whom his eider brother sided in the Satsuma insurrection of 1877, but he nevertheless remained loyal to the imperial cause and commanded a brigade against the insurgents.
In 1870, he went over to France to study and observed the Franco-Prussian War. The following year, he returned to Japan. After promotion to army major general, he went to France again for further study.
On his return home, he engaged in the construction of the army. He served as commander in chief of the Detached First Brigade during the Satsuma Rebellion. Later, he successively held posts such as vice-chief of the General Staff Office, rikugunkyo(War Minister), and Minister of War from the first Ito cabinet to the second Matsukata cabinet. During this period, he became an army general and privy councillor. During the Sino-Japanese War, he was treated as a genro (elder statesman) and became commander of the Second Army. In 1898, he became marshal, and chief of the General Staff in the following year. During the Russo-Japanese War he served as general commander of the Manchuria Army. In 1907 he received the title of koshaku (prince), and from 1914 he served as Lord Keeper of the Privy Seal.
A protégé of Okubo Toshimichi, he worked to overthrow the Tokugawa Shogunate and thus played a major role in the Meiji Restoration.
He served as the commander of the Detached First Brigade during the Boshin War. At the Battle of Aizu, Ōyama was a commander at the Satchō Alliance's field artillery positions on Mount Oda.
Oyama Iwao contributed to the foundation of the Japanese Army and went to the front in the Sino-Japanese War and the Russo-Japanese War. He received the British Order of Merit in 1906. From 1914 to his death he served as the Lord Keeper of the Privy Seal. His Japanese decorations included Order of the Golden Kite (1st class) and Order of the Chrysanthemum.
Ōyama was a large man, and enjoyed large meals. His weight exceeded 210 lbs. , and may have contributed to his death, possibly arising from diabetes.
Ōyama's first wife Sawa died of puerperal disorder. Second wife Sutematsu (a survivor of the Battle of Aizu, a sister of former Aizu retainers Yamakawa Hiroshi and Yamakawa Kenjirō) was one of the first female students sent to the United States as part of the Iwakura Mission in the early 1870s. She spent eleven years there, graduating from Vassar College in 1882.
In the next year she accepted her former enemy's proposal.
Ōyama was Emperor Meiji's first candidate for rearing future emperor Hirohito as a sort of surrogate father in 1901, in accordance with royal customs, but Ōyama declined and the role instead went to Count Kawamura Sumiyoshi.
Ōyama's first son Takashi, a navy cadet, died in the accidental explosion and sinking of the cruiser Matsushima in 1908. Second son Kashiwa (ja) became an archaeologist after he retired from army.
field marshal
