Not really. The O2 has been publicly verified :
http://www.innerfidelity.com/content/innerfidelity-headphone-amp-measurement-program-update-july-2012
In consideration of that, neither is there reason to doubt the designer's ODAC measurements.
The only thing that might be an issue is if one has extremely power-hungry headphones where distortion might occur but more likely just be quiet. But there are precious few of those. The HE6 is thought to need more power than the O2 can provide.
Sonically reviewing the O2 and declaring another amp 'better' is only to reveal a lack of understanding of what the O2 is all about (other than in the power department, of course). It's not just an amp: it's a demonstration that hifi can be had for not much money and that people are being ripped off.
If the other amp is also a perfect hifi source, which any properly designed amp should be, then neither would be better.
One of the marvels of the clip+, for example, is that it is almost hifi. It has a tiny bit of almost inaudible noise but that's about it. A clip+ with O2, or the O2/DOAC combo for a PC/MAC gets you hifi everywhere.