Not only is the Asgard 2 great sounding headphone that runs in true "Class A" but it also makes for a wonderful sounding preamp with High and Low Gain settings. You may not need a preamp now, but you never know when that built-in flexibility/functionality may come in handy. I'm sure that the O2 is a great sounding amp, but I prefer to have the input and preamp outputs connections on the rear of the case. Obviously, this is a Schiit Audio Asgard 2 thread, so I readily admit my preference for Schiit Audio components. Having been in this hobby/passion since the early to mid 80's, I've come to truly appreciate it when I see a company like Schiit Audio that offers components that consistently outperform their price point. Plus, they let the gear do the talking rather than make outlandish claims regarding their performance.
Good luck with whatever you decide!