I'm not going to BS you by telling you that there's a ton of difference between the Green Pony and the OD2--after all, they are both based upon the Tube Screamer core. Normally, I'd point out the fact that the Green Pony can be run as a clean boost, something that a TS doesn't offer (though the gain stage can be modified to get pretty close), but since the OD2 incorporates a separate boost stage in the same pedal enclosure, that's really not relevant in this particular comparison.

Still, there are significant differences:

* The silicon diode clipping setup in the GP is asymmetric, so it sounds a bit warmer & smoother than the OD2. The overall gain of the overdrive stage is the same as a Tube Screamer or the OD2.
* The GP has more overall output volume. It uses charge pump technology to double the internal voltage to 18V, giving much more headroom to the clean boost and a ton more push to your amp.
* The parallel clean boost and overdrive paths are a cool feature, so as you turn up the dual gang Drive pot, it blends less and less of the clean and more and more of the OD. And as explained on the BYOC product page, it does it much more efficiently than a Klon, since signal is not being dumped to ground to achieve this.
* The biggest difference to my ear is that the GP has much less of that "mid-hump" that TS pedals are well known for, and combined with other differences in the EQ structure of the pedal and the 18V power stage, it ends up with a noticeably more open and neutral sound. It just sounds BIGGER. More so than the Klon, as well, which also has a noticeable mid-hump, though not as pronounced as a TS.

So, bottom line, if you have one of my OD2's, I wouldn't say that you NEED a Green Pony. But that's not to say that you might not really enjoy one, 'cuz it's a great sounding boost & overdrive, and stacks well with the OD2. I'd put it behind the OD2, to get the full benefit of its 18V capability.