You can learn more about the new Japanese heavy tanks here!
The O-I can be equipped with two excellent guns: a 100mm gun in its stock configuration, and a 150mm Howitzer available through research. While the stock gun is effective, it's the top-tier gun, similar to the 152 gun of the Soviet KV-2, that truly makes the O-I a formidable opponent. It's most effective at close and medium range, and lets the O-I knock out most tier VI tanks in a single hit. All because the Howitzer's penetration damage is practically in the same level of damage dealt by artillery!
The O-I boasts 150mm of rear and frontal armor; impressive for a tier VI vehicle. Even if a shot gets through, this Japanese behemoth has an incredible 970 hit points, larger than all of its peers (well, except for the Premium TOG II*).
The O-I has good mobility relative to its large size, with a maximum speed of 30 km/h, and quick acceleration to a cruise speed of 24-26 km/h. It can also "dance" a bit, with a traverse speed of 22 degrees per second.
Even without additional equipment mounted, the tank's view range is 370 meters, which enables it to see enemies at a distance.
Among other tier VI heavy tanks, the O-I is a giant, which can make finding cover and concealment a real challenge. While most bushes aren't suitable for concealing the O-I's enormous hull, such a design has its advantages -- the high placement of its gun allows it to hit the weak spots of shorter vehicles (such as the thinner armor of their turret tops) easily in close combat.
To play the O-I effectively, you will need to know when to switch between HE and AP shells. When you are matched high on the team list, you will have more time and leisure to choose your targets and let your aiming circle settle in. You may be better off using HE shells for long-range firing, as the Howitzer will not be able to hit weak spots at a distance of 200 meters and more.
If the tank is in the middle of the list, it is a good idea to be more careful -- Make HE shells your primary choice and give close support to your allies.
In tier VIII battles when you place lower on the list, you will want to survive as long as possible while keeping your gun in the game. Avoid open spaces, follow the rule of "hit-and-run" and travel with groups of allies to disperse enemy fire. Your armor can block the shots of several Tier VIII vehicles, but you don't want to take chances with enemy fire. Wait for the "big boys" to engage enemy tanks first, then pop out and deal damage when the enemy is distracted.
In some respects, the O-I is like an E-100 at tier VI: A huge, well-armored tank that can do high damage with a single shot!