In 1910, Okamura entered the 25th class of the Army War College, and was promoted to captain soon after graduation in 1913. He served in a number of staff positions on the Imperial Japanese Army General Staff during and after World War I including an assignment to Beijing in June 1914. From June to December 1921, he was sent as a military attache to the United States and Europe. While at Baden-Baden in Germany, he met with Tetsuzan Nagata, Toshiro Obata and Hideki Tojo, laying the foundation for the "Tōseiha" political clique within the Japanese Army. On his return to Japan, he was assigned to the 14th Infantry Regiment.