Rubuta ci gaba don wannan sakin layi - A cikin 1900, an ba Ōkubo mukamin Laftanar Janar kuma shugaban ma'aikatan Babban Jami'in Horar da Sojoji. A watan Mayu na 1902, ya zama kwamandan rundunar soji ta 6 ta IJA. A lokacin Yaƙin Russo-Japan, an haɗa Rukunin Sojoji na 6 na IJA da Sojojin Japan na Biyu, amma bayan Yaƙin Shaho
An canza shi zuwa rundunar sojojin Japan ta hudu kafin yakin Mukden. Bayan yakin, an ba shi lambar yabo ta Order of the Rising Sun, aji na 1 da kuma Order of the Golden Kite, aji na 2. A watan Yulin 1906, an canza shi zuwa kwamandan IJA na 3 kuma a watan Satumba na 1907 an daukaka shi zuwa matsayin "kazoku" tare da taken "danshaku" (baron).