Ōishi Masami (大石 正巳, 26 May 1855 – 12 July 1935) was a politician and cabinet minister in the pre-war Empire of Japan.

In 1873, he joined with Itagaki Taisuke and became an important member of the Freedom and People's Rights Movement.

Under the short-lived 1st Ōkuma Shigenobu administration in 1898, Oishi was appointed Minister of Agriculture and Commerce.

Ōishi later joined the Rikken Kokumintō and was at one point a contender against Inukai Tsuyoshi for its leadership.

In 1913, he broke with Inukai, and joined Katsura Tarō’s new Rikken Dōshikai, where he was ranked as one of its five leaders.