Does your O2/ODAC combo have a Line-Out or just the Headphone out?
And you say coaxial...what exactly do you mean by that? You need to feed the A-100 an analog signal via 2 RCA's (Red and White cable).
A direct dac-out or line-out would be the best way to pass the signal to the amp, skipping the internal amp in the O2 (and avoiding dual amping). But going from the 3.5mm to dual RCA would still work. You would essentially be using the O2 in a sort of 'pre-amp' fashion where the volume would be controlled by both the O2 and A-100.
I'm doing something similar at the moment where I have the Line-Out on my Matrix M-Stage going to the input on the A-100. I just set the A-100 to about 9 o'clock on the pot and control the volume via the M-Stage. Works well. As I mentioned, a Line-Out would be better than the headphone out if it is a possibility.
It just has the headphone out :( I knew as soon as I recieved it that I should have gotten the O2 and ODAC separately. I have read about people having success using the O2 as a preamp, so I figured it probably would work. And yeah, by coaxial I meant RCA, sorry for the confusion. If people recommend against it, I do have my trusty FiiO E10 which is still IMO a respectable DAC and does have a dedicated line out as well as a coaxial out, so all I'd need is a pretty inexpensive coaxial to RCA adaptor and I'd be off to the races...at least until I decide to pull the trigger on a fancy new DAC.