Born in Tokyo in 1884, Okamura enrolled in Sakamachi Elementary School and graduated eight years later. In 1897, he entered Waseda Junior High School. In 1898, he was transferred to Tokyo Junior Army School, and was transferred to Army Central Junior School later. Okamura entered the 16th class of the Imperial Japanese Army Academy in 1899 and graduated in 1904. His classmates included the future generals Itagaki Seishiro, Kenji Doihara and Ando Rikichi. He was commissioned a second lieutenant in the IJA 1st Infantry Regiment. In 1907, he was promoted to lieutenant and was assigned to the Army Academy to assist with the training of cadets from China.