When you have a O2/ODAC combo, the space for the 2 9V batteries to power the O2 is sacrificed to host the ODAC PCB. So to use it, you will always need a wallwart. Personally, I find this a little impractical, as I use my O2/ODAC with my laptop, and sometimes I find myself in situations where I want to use it, but have no power outlet available.
Through the DIYaudio forums I found a solution where they simply moved the ODAC pcb (after insulating it with PVC tape) to the space between the O2 pcb and the bottom of the case and then drill a new hole in the rear panel of the case for the mini USB connector. While this is really quite simple, it is also (IMHO) pretty ugly. The original USB hole will be there being useless, and the new hole will never be as nicely finished as the professionally machined original.
Also, I thought about making an external batterypack in a matching case. But this would also mean I needed to either modify the original casing with connectors for the external power, or make a overly complicated batterypack that puts out AC so I could use the existing powerjack.
So, I was thinking about a slightly different approach. If the ODAC pcb is flat enough to fit between the O2 pcb and the bottom of the case, it really only takes up a fraction of the space for the battery, so why not look for a more compact battery? 9V batteries do not come in a lot of sizes and shapes, and when it comes to compact, the standard 9V block is pretty much the smallest you will find.
That's why I'm now thinking of building a custom batterypack, but even 2 6xAAAA cell flatpacks are hard to fit in the case. 1/2 AAAA cells will work, but their capacity is usually only around 180mAH.
But Li-Po cells are available in crazy thin formats these days. And capacity is usually a bit better. So why not make 2 3-cell ultra-flat Li-Po packs, which could probably fit in the space between the O2 pcb and the bottom of the case. Ofcourse they'll put out a few too many volts, but above the ODAC pcb there is some space left for a little DC-DC converter or regulator circuitry... I know, it's a little bit complex and Li-Po does have some risks when it comes to charging, but I'd still like to hear some other peoples opinions on this idea :)