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|Place of origin||Japan|
|Designed||1939–1940 (100 ton O-I),
1944 (120 ton O-I)
|Produced||1940 (100 ton O-I), 1945 (120 ton O-I)|
|Number built||2 (one 100 ton O-I prototype, and one 120 ton O-I prototype)|
|Armor||maximum of 200 mm (120 ton O-I)|
|105 mm gun with two light rocket artillery canisters|
|37 mm Type 1 gun, 3x 7.7 mm Type 97 machine gun (120 ton O-I)|
|Engine||Two V-12 gasoline engines
O-I was the name given to a proposed series of Japanese super-heavy tanks, to be used in the Pacific Theater. The vehicle was very heavy, carrying 11 crew in its 100-120 ton body. Only one 120 ton O-I model was rumored to have been built in 1944 and afterward sent to Manchuria. Exact information is lacking however, and it is unknown whether it ever saw combat. Whereas the original plans called for three turrets for the one large cannon and two smaller guns, a more advanced experimental prototype, the Ultra-Heavy Tank OI featured no fewer than four turrets.
After the Battles of Khalkhin Gol with Soviet Union in 1939, Japan tried to improve its tank design using lessons learnt from this battle. Many Japanese tanks such as Type 95 Ha-Go light tank and Type 97 Chi-Ha medium tanks were proven to be insufficient to counter Soviet armored forces. Larger tank design was urgently needed. A super heavy tank project was proposed along with new medium tank designs.
In 1940, Hideo Iwakuro Colonel from the Army Ministry of Japan (陸軍省 Rikugun-shō) ordered the Army Engineering Division to develop a new super heavy tank. Iwakuro Colonel also indicated that the new tank should be at least two times larger than the current Type 95 Heavy Tank(26 tonnes).
The fifth Army Engineering Division carried out the project secretly and completed a prototype by the end of 1940. The 100 tonne prototype was equipped with a 105mm cannon and could carry over one hundred shells for its main gun. Its secondary armament consisted of a 75 mm cannon and a 7.7 mm machine gun.
The frontal hull of the tank was protected by a 75 mm thick armor and another 75 mm steel plate could be attached to provide additional protection. The side armor of the tank was only 35 mm but it could also attach 35 mm steel plate if needed.
During test trials, engineers realized that it would be highly challenging to deploy this tank. The weight of O-I tank added too much pressure on its suspension system and made it impossible to pass through uneven terrain. The prototype was therefore left in the factory and was scrapped in 1944 and its blueprint was later lost in the war.
Inspired by Germany's super-heavy tank "Maus", Japan restarted its super heavy tank project. Mitsubishi Heavy Industries Tokyo Machinery Division started the development process in 1944. The alias of this new tank was "O-I tank" (オイ車, "オ" in Japanese this means "large" and "イ" means "one").
O-I tank mounted a modified version of Type 92 10 cm Cannon and a Type 1 47 mm tank cannon on its main turret. Besides the main turret, it also had three diagonally-mounted secondary turrets with one Type 97 7.7 mm machine gun on each one of them.
One of the main features of O-I tank was its thick armor. Its main turret had 200 mm all-rounded armor as well as the frontal hull. O-I had 35 mm +75 mm on its side and 150 mm thick armor at the back. Compared to the Type 5 Tank Destroyer Ho-Ri developed in the same year (with only 125 mm armor), the armor of O-I was far superior.
Total weight of O-I tank was over 120 tonnes which set a new record for Japanese military industry. A new suspension system and a pair of 750 mm wide tracks were developed to support this massive tank.
One prototype vehicle was successfully completed during the last year of war. When the war ended in 1945, the tank was being transported to Manchuria. It was later scrapped with only its tracks left. The tracks of O-I tank are now on display at JGSDF Fuji School in Japan.
In Popular Culture
The O-I was featured in the video game R.U.S.E. as part of the downloadable content Japanese faction. It is also featured in the PC game World of Tanks as a playable vehicle belonging to the Japanese heavy line. Other tanks belonging to the same line included different versions of the O-I project under altered names.  According to Wargaming.net, these tanks did exist (albeit in blueprints only), although no other sources can confirm this.