Well, I can tell you one thing. It's not Overpowered.
But, I'd be lying if I didn't think the Octobrush was OP when I first played this game. For the first couple months, I swore that the Octobrush turned into Overpowered Garbage in this game. However, looking at what they changed about the Octobrush, nothing really changed. The only change they made was that the base damage was increased from 37 -> 40, making it still a 3 flick kill (most of the time).
But why did I find this weapon brutally was Overpowered at the time? And that's just because of the brush's playstyle. Camping / Sharking, waiting for an opponent to come close to you and then go for a splat. What makes the brush so good at this? The fact that it requires no aim while up close. Seriously. The Octobrush does not require the user to be at least decent at aiming. The Brush hits everything in front of you, and it can even hit people on your sides rather reliably.
All a brush player needs is patience (wait until the enemies are in your range) and some paint to support.
Making it a wonderful weapon to ambush with since the damage spread on the weapon is generous enough that missing almost never happens up close. And consistently flicking 3-4 times (5 with falloff) will kill most of the time.
It got buffed and nerfed I believe. Contrary to popular belief this thing actually has one of the fastest kill times in the game, it is just hard to pull off. There is a sweet spot that when you swing the brush it can hit your oponent multiple times, but hitting this spot is hard with moving opponents. Also it can run away if it wants, so that is always good.
That mechanic got removed from the Octobrush from Splat 1 to splat 2, so you can't two flick anymore to kill. And even with the critical damage mechanic, it was rather difficult to pull off even in the testing range, let alone with a moving target and an uncontrolled environment
But what makes an Octobrush successful is trapping the aggressive opponent constantly up close, since it's rather easy for a brush to kill like this. And since western players follow the stereotype of being aggressive and not painting much, the octobrush performs extremely well in this situation.
In Japan where they tend to turf the map much more (allowing the octobrush less spaces to shark) the octobrush's tactics don't work as much there. Since the octobrush is constantly locked out and it can't do it's magic against players who play defensively.