It can select from analog or digital input via a selector switch on the front panel. This is a 4-pole double throw switch which switches between RCA jacks on the back panel and the analog output of the DAC.
It can select line-level or headphone output via another 4-pole, double throw switch, also on the front panel. This switch simply takes the signal from the input switch and sends it to either an other set of RCA jacks or to the O2 headphone amplifier.
Note that everything on the front panel of the O2 has been replaced with its panel-mount equivalent, and moved to the front panel of my enclosure. This creates quite a tangle of wires crammed into a small space. Hooking everything up was more work than was building either the DAC or the O2. The last image is a PowerPoint file I made to help myself get all the connections right.
I'm using a transformer built into the case for the O2 power supply.
Also note the inconvenient use of two separate volume control knobs - one for the headphone out and one for the line-level out. I wasn't able to find a good way to attenuate both of these signals with one dial.