Kammala jumla mai zuwa: A shekara ta 1900, an daukaka Ōkubo zuwa 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 da aka sake dawo da su, an sake mayar da su zuwa rundunar soja ta biyu.
An canja wurin yaƙin Shaho zuwa rundunar sojojin Japan ta huɗu 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).