Okabe was born in Hiroshima city and attended military preparatory schools as a youth. He graduated from the 18th class of the Imperial Japanese Army Academy, where his classmates included Tomoyuki Yamashita and Korechika Anami. He subsequently graduated from the 27th class of the Army Staff College. In 1918, with the rank of captain, he was sent to the Vladivostok Special Operations Office during the Japanese intervention in Siberia, and as a major in 1922, he was sent as a military attache to Poland. During his time in Poland, he acquired the latest code encryption technology from one of his contacts in the Polish General Staff.