Oyama Iwao (1842 — 1916) was a Japanese field marshal, and one of the leaders who led the Meiji Restoration.

He was born in Satsuma Province, which is now Kagoshima. He was a nephew of Saigo, with whom his elder brother sided in the Satsuma insurrection of 1877, but he nevertheless remained loyal to the imperial cause and commanded a brigade against the insurgents.

When war broke out between China and Japan in 1894, he was appointed Commander-in-Chief of the second Japanese army corps, which, landing on the Liaotung Peninsula, carried Port Arthur by storm, and, subsequently crossing to Shantung, captured the fortress of Weihaiwei.

For these services he received the title of marquess, and, three years later, he became field-marshal. When (1904) his country became embroiled in war with Russia, he was named commander-in-chief of the Japanese armies in Manchuria, and in the sequel of Japan's victory the emperor bestowed on him (1907) the rank of prince. He received the British Order of Merit in 1906.