Comparing: O-I vs. T-150

Development of the superheavy tank was started after the Battles of Khalkhyn Gol in 1939. The vehicle was designed as a maneuverable fire unit that was unprecedented in scale—its hull alone weighted 100 tons. Only one prototype was built, without a turret and made of construction steel. Trials were discontinued due to the unreliable engine; at the end of 1944, the prototype was scrapped. The 15-cm howitzer was to be mounted on the tank after the assemble.

The T-150 was a further development of the KV-1. The vehicle weighed as much as 50 tons. The T-150 underwent trials in the first half of 1941. A prototype fought in the battles for Leningrad, and became a basis for a modification of the KV-1 with reinforced armor.