Unfortunately, the Japanese HT line is extremely bad and might seriously be the worst line in the entire game.
The O-I, which I assume is the only one you've played so far (as you're getting the O-Ni), is actually indeed a solid tank. You gotta give the O-I some serious respect - the tank has been nerfed like what, 3 or 4 times, on TOP of those numerous nerfs the HE rework also happened, yet despite all that, the tank still manages to be decent. Sure, it's not completely busted as it was in 2015-2016, but it can still be a fun tank. The derp gun can still work because the damage is just that absurd, and the AP and HEAT are actually perfectly workable, so you dont need to rely on spamming HE (which is very unreliable). The armor is easily defeated with premium, yes, but you gotta also remember that most tier 6 heavies have fuck all armor even against standard ammo, so its still very strong.
Unfortunately, the line goes very quickly downhill. The O-Ni isn't bad, but it's not very good and it's considerably less fun than the O-I. The derp gun is no longer viable, so you have to use the AP gun, which is workable, but pretty bad and not very exciting. What saves the O-Ni is the HP pool which is absolutely bonkers for a tier 7 heavy. With IH, you can get almost tier 9 levels of HP. The mobility with turbo is surprisingly not that awful because how strong the engine power is. Other than that, it's still plagued by same issues as the higher tier Japanese HTs.
The O-Ni isn't great, but it isn't awful or anything. Then we get the O-Ho. Jesus. There's a massive, massive list of tier 8 tanks (albeit most of them are premiums...lol), and the O-Ho is a contender for the absolute worst tier 8 in the entire game. It's huge and slow. The armor is...pretty mediocre. It's not even amazing against standard ammo, as a lot of heavies and even some mediums have a 50/50 chance to pen it on a lot of spots, and then gold ammo obviously completely trashes it. The derp gun is completely unusable. The AP gun on the other hand, is a pretty shit gun...but it very, very heavily contradicts the platform its on. For a huge, fat, slow tank with shit armor for its type, you'd want a gun with either very high alpha damage or exceptional gun handling so you can expose yourself as little as possible. The AP gun is complete opposite, as it has sub-par alpha and poor gun handling...so yeah. The O-Ho has awful gun depression over the miniturrets. And to top it of, its on a same tier with all the broken premium tanks like the Skoda T 56. A complete carcrash of a tank.
The Type 4 and 5 are both just huge, slow boxes with trash guns that are asking to get gold spammed. The Type 5 is maybe a little better than the Type 4, and both are better than the O-Ho, but despite that, they're still the worst HTs in their tiers. The Type 4 has a bit better guns for its tier, as 600 / 680 alpha damage is absurd at tier 9, but the armor is very wonky and can be frustrating even VS standard ammo. The Type 5 has better protection (still gets gold spammed to death), but the guns are more underwhelming.
The best option for you to is to stop at the O-I and keep it. If you enjoy the gameplay of the O-I...you won't get similar gameplay at higher tiers, unfortunately. I personally enjoy the Type 5 Heavy...sometimes, but I'm a masochist and I have very fond memories of the Type 5 ever since the beginning, so the tank has a place on my heart. Most people aren't like me, and if you want to play a tank like the Type 5, just get the E 100 instead. It's basically a better version of the Type 5.
TL;DR: Keep the O-I, abandon higher tier Japanese HTs, and get the E 100.