 The complete history of the O-I is unknown, due to the "obscure" nature of the project and the limited documentation known to have survived post-war.After the Battles of Khalkhin Gol against the Soviet Union in 1939, Japan tried to improve their tank designs using lessons learned from this battle.Many Japanese tanks such as the Type 95 Ha-Go light tank and the Type 97 Chi-Ha medium tanks were proven to be insufficient to counter Soviet armored forces.