Extremely close, but O2 has higher output impedance from the first listen unless implemented perfectly.
I've seen O2 being 0.5, 1.1 and 1.9 Ohm, sample size of 3. (nvawguy reference, JDS Labs Joker's and the local home made.)
UHA6 mkII has immeasurable impedance. That is, my 1% precision voltmeter with added external precision Kelvin bridge with 1% reference resistors cannot measure its impedance. (Specifically, it's below 0,05 Ohm.)
I cannot speak for ODAC, as I haven't ever heard it. From what it looks, it is an extremely competent DAC, but the Cirrus Logic implemented in Leckerton is implemented by one of the designers of the chip...
The limiting factor of Leckerton via USB is the TI interface chip. Not a huge limitation if you don't need digital volume control in the DAC - the noise floor of 16-bit is not audible unless ridiculous gain is applied. (e.g. 24 dB)
Just make sure the OS and any DSP you use is better than 16-bit and the downmixing is done correctly, preferably with dither. For example, Pulseaudio and Windows 8 dither (rectangular half bit), while Windows 7 and ALSA don't.
It's best to use SPDIF for DAC duties instead though (preferably optical) and external high quality power supply. AFAIR ODAC doesn't have SPDIF, so cannot do this.