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 monstrous, carrying 11 crew in its 100-120ton body, but only one 120ton 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. It is unlikely that the Ultra-Heavy was ever actually built.