I agree with the 840c being too bright for the O2. With the 717, which has a warmish tonality but not too warm, the 840c was metallic, dry, and full of digital glare. It did sound very airy and detailed, but it also didn't have enough PRaT, and was distinctly laid-back. It worked pretty well for very fluidly recorded ambient electronica, where it was very airy, and for some large-scale classical pieces where the glare actually added a layer of sparkle to the sound. But for most things, it was way too colored and in a bad way. The O2 is just too damn revealing.
Still, it's still a decent source with the O2 for the money, since most things in the price range don't have anywhere near the necessary resolution and end up sounding grainy - which the 840c certainly didn't. I'll take its colorations over hideous grain any day.
The Opus 21 is better with the O2 in every way, but can also sound dark and dull depending on cabling.
Actually, when you get down to it, the O2 is very similar to the HD650 in that it has the power to sound truly dreadful in many systems. To make matters worse, the difference between good and dreadful can be as simple as an interconnect. The O2 is not a foolproof headphone, in fact it's probably the least foolproof headphone I've encountered. It needs you to tune your system by ear, since it will reveal everything about system synergy, but at the same time in needs the amp to have enough power to open it up and control the bass. The system has to be neutral, and it has to not have any serious imperfections and colorations. That's actually pretty hard to put together regardless of budget if you don't take the time to swap components in and out to see what works best. You absolutely cannot throw the O2 into any old system and expect it to work off the bat.
Subtle colorations are a good thing, IMO. They give you much more room to work with. Even the HD650, which is revealing enough in its own right, is much easier to system match properly. The O2 is too neutral for most consumer and a lot of audiophile-level gear, and you really have to be very picky. But on the upside, you can tune it to have different sound signatures depending on the rest of your system.