As a frequent player of the "assassin" Oak, I have to say that he is balanced, but may shine in certain ways that are frustrating to the point of someone throwing up their hands and saying "How the fuck do I beat this?" Let me elaborate.
1.) Oak assassin's effectiveness is partially map-based. As other people here have said, Oak's burst, while not horrible, isn't what wins him fights. Oak kills people by latching on to them and never letting go, taking a beating while killing them, if necessary. On certain maps, this is incredibly easy to do. For example, let's take prison. Prison has a ton of towers in each base, but the "no-man's land" part of the map has NONE. This makes it stupidly easy for Oak to chase someone indefinitely. Having been on the receiving and dishing end of this, I can't see how to really stop a smart Oak player from killing certain DG's (squishy, low-hp) on prison unless said DG sees him coming from a ways away and RUNS IMMEDIATELY. However, on maps like Cataract, this is much much harder to do, and often ends in a killswap, if anything. So you can't really blame Oak for being OP on certain maps; it's the same situation as Rook on Crucible.
2.) Oak's killing power revolves around an enemy DG letting himself get too close to the brink. Think of it this way: if you're fighting against one or more Regulus who are heavy snipe build, you know not to let yourself get below a certain amount of hp. The same thing applies to Oak, but people have trouble accepting this fact. Let me give an example. An enemy Rook drops down to about 1.5k-1.7k hp, and I have second level pentinence. If that Rook does not retreat IMMEDIATELY upon seeing me, he will die. 400dmg penti + 2 slashes, and you're at less than 700 health, and I'm on top of you. The thing that whiners need to realize is that when an Oak is on the map, you need to play things differently. Oak's kills rely on that Penti + latching on to you, despite being dragged through fire. You just need to adjust accordingly. That Rook could have taken measures that I, as the Oak, can't be immediately aware of in order to survive. He could boulder roll me and run. He could have wand of speed and muscle through my pentinence before I can actually start hitting him. People just don't know how to adapt because they're used to preparing for suddent burst damage as opposed to hard-to-avoid sustained damage.
3.) Assassin Oak's are probably the most speed-dependent DG. This works both ways - denying them speed is often more painful than any kind of damage. Let the Oak rush in after you, deep into your towers, then do something to bind his movements and he will likely die on the way out.
These are just what I can come up with off the top of my head. Just try to remain open-minded with the Oak, as you should with any DG. There are counters for everything, but they may require you to play differently than you are used to. This does not mean they are OP, it just means that they're doing it right and you're not.