Reviewing this as someone who has extensively played the Japanese heavy line since their release in 2015, and 3 marked every single Japanese heavy tank.

Review is based on patch 1.17.1.

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Pros:
- The highest HP pool at tier 7 (1550)
- Strong frontal armor that is resilient to many tier 7s and most lower tiers
- Meaty alpha damage (330) and decent standard ammo penetration with 10 cm gun
- 15 cm derp gun can deal devasting damage if it penetrates
- Good gun depression (10 degrees)
- Surprisingly good engine power, giving the O-Ni disproportionately good acceleration for a super-heavy
- Great view range for a tier 7
- Thick rear armor that can bounce tier 5s and many tier 6s


Cons:
- The single tallest tank in the game, making the O-Ni a massive target and also granting it the absolute worst camo rating in the game
- The armor can be penetrated pretty easily by premium ammo and tier 8-9s
- Extremely weak, flat, huge side armor. Can't sidescrape very well
- Miniturrets limit gun depression, though this issue isn't very prominent with the 10 cm gun.
- Still very slow despite its great acceleration, as the top speed is capped to 25 km/h
- Awkward turret placement that's not very well suited for peeking corners, while the tank itself is poor for sidescraping
- 10 cm gun has poor DPM, very long aiming time, bad accuracy and mediocre premium ammo
- 15 cm derp gun has terrible accuracy, penetration and the HE shells are not very reliable. Generally, its nearly completely useless against higher tiers and while not unusable at tier 7, its not a recommended choice
- Huge ammorack that gets damaged easily

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Equipment: Improved Hardening / Gun Rammer / Turbocharger

If you're willing to gamble for a good matchup and use the derp gun, a more dedicated loadout with stuff like Improved Aiming Unit is possible, but I'd rather still use the standard Hardening / Rammer / Turbo loadout even with the derp gun.

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The review itself:

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The O-Ni is a followup to the O-I. Unfortunately, this is where the steep downhill starts. While most of the Japanese heavies already fell off even before the HE rework, the O-Ni was still a pretty solid tank not too long time ago. It was one of the few cases where the HE rework truly hurt it - the O-Ni does not have a high-penetration HE to benefit from HE rework, its alternative gun is very mediocre, and it didn't receive any sort of compensation buffs. That being said, even now, the O-Ni isn't a truly terrible vehicle like most of the Japanese heavies are. It being a tier 7 helps a lot, as its not nearly as a hostile environment as higher tiers. However, it's a pretty mediocre vehicle and can definitely feel disappointing after the O-I, as the derp gun isn't even half as appealing anymore. The worst part is that the player will learn to appreciate the O-Ni a lot, because it's the last tank on the Japanese heavy line that's not a good contender for a worst heavy tank in its tier. Tier 7 being infested with a lot of crap tanks also helps the O-Ni, as its still better than quite a few tier 7 heavies.

As a Japanese heavy, the O-Ni comes with some obvious strengths like rest of them. As you'd expect from arguably the single biggest tank in the game, the O-Ni has insanely high HP pool. 1550 is higher than many tier 8s! This gets even better, as with Improved Hardening and Field Modifications, the O-Ni can get its HP to well over 1700s, higher than some TIER 9s like Emil II! O-Ni has so much health that even if its armor doesn't work, it often can do stuff. Obviously, the O-Ni has strong frontal armor that lets it stomp tier 5s, 6s, and many tier 7s with ease. 150 mm rear armor also helps with your status as a lower tier bully. The O-Ni has one unique strength - deceptively good specific power at around 12 hp/ton. This is due to its surprisingly light weight of "only" 100 tons, and it retaining the powerful 1200 hp twin engine the rest of the Japanese heavies use. This gives the O-Ni very good acceleration to the point where it reaches its top speed almost instantly. With Turbocharger, the O-Ni isn't...THAT slow. It's slow, but because you can still accelerate to 29 km/h almost instantly, it's not the worst thing around.

The O-Ni comes with two guns: 10 cm and 15 cm derp gun. Unfortunately, neither are great, though as of HE rework, the 10 cm is definitely a superior choice. The 10 cm is nothing special - a fairly mediocre 10 cm 330 alpha gun with crap DPM and accuracy that is very comparable to many similar guns at tier 7, though the O-Ni has really, really bad aiming time even for its type. The 15 cm derp is more interesting choice, but it falls a bit short. The main problem is the penetration. Unlike the O-I, on which the AP/HEAT work fine most of the time at tier 6, 121/150 mm penetration is not a very good penetration to have at tier 7. The derp gun can still be pretty nasty when top tier, but with the HE rework, it's almost completely useless when the O-Ni isn't top tier. It's a bit of an all-or-nothing type of gun, where you're willingly gambling for top tier matchups. If you're playing the O-Ni for memes and you're looking for something different, though, the derp gun isn't completely unusable, so you can go for it. The satisfaction of getting to destroy things when you finally get that good matchup can be there. Make sure to use Intuition skill for your loader, you REALLY need it. Otherwise, I heavily recommend using the 10 cm gun. The O-Ni is still a beast when top tier even with that gun, and makes it at least a bit better than "completely useless" when its not top tier.

The O-Ni is a simple tank to play. You use your huge HP pool to brawl and trade. You're better off to peek corners with your front exposed; the side armor is very poor, so you can't sidescrape or angle very well. When not top tier, be a little bit more careful. Remember, as a fun little trick, your rear armor is very thick, so if there's someone with very low pen behind you, you don't necessarily need to turn your entire tank around to deal with them. It's mostly going to happen when top tier, as flat 150 mm armor isn't THAT hard to deal for most tanks the O-Ni faces. Be aware that premium ammo and higher tiers completely wreck your armor, you're literally playing the tallest tank in the game so hiding can be a nightmare, and your guns are overall some of the worst in tier. The O-Ni is a very flawed vehicle, so you kinda have to expect inconsistent performance.

Overall, the O-Ni is definitely worse than the O-I, and starts the unfortunate downhill towards the higher tier Japanese heavies. That being said, the O-Ni isn't awful. It's definitely more functional than the higher tier Japanese heavies. It still has some meme potential as it can be a menace in a good matchup, but it's overall a pretty sub-par vehicle that's flawed in many ways. It's not really a keeper - if you're looking for a somewhat decent Japanese heavy to keep and don't want to bother with actually bad higher tiers, you should've stopped at the O-I instead.

Good