[ONE Accessories] Reignogleph MMXI:
Here's a list of DACs reportedly working with the OPO:

The Audioquest Dragonfly works with USB Audio Recorder/Player Pro (as long as Tweak 1 is installed, according to chazman1117)

The FiiO E07k, according to this OnePlus One Forums member.

The Fiio E10 actually works with "an OTG cable and no external power," as reported by benoitb and confirmed by me! The E10 doesn't require UAPP/UARP to work; it receives signal from every audio player I've used, and EQ utilities also have a direct effect.

But since I've read that using an unpowered USB DAC with an unpowering source can damage said source, I don't think I'll be making a habit of using an E10 with my OPO -- especially in light of this week's exploding battery fiasco.

The FiiO E17 works well with the CyanHacker ROM and possibly stock CM as well (according to sandels' findings)

The FiiO E18 (confirmations appear here and here, though it's a tad unclear whether the latter poster's talking about an OPO or a "graveyarded" Nexus 5)

The FiiO E7 is working with the OPO, according to Head-fi's Cattlethief.

The Apex Glacier DAC, Poimandres of Head-fi assures us, not only works quite well with the CM's 38r 24/192-ready update but "streams USB audio from Neutron divinely." (Apex wants you to know that the Glacier uses a "bipolar power supply" -- best to listen to demanding headphones when it's not in the manic phase, apparently!)

The HiFiMeDIY TinyDAC works with KitKat but not Android L, reportedly; I know from personal experience that it works with an OPO running CM11 stock.

The HRT Music Streamer II is working with the OPO, according to Head-fi's Cattlethief.

The iFi Nano iDSD is reportedly working with the OPO according to this post by Head-fi member cattlethief.

The NuForce uDAC-2 is reportedly working with the OPO according to this post on Head-Fi (to which DanBa was kind enough to drop in and link).

The Rega DAC works with the OPO without any issues according to mapin0518, who used it with Google Play Music and this Micro USB to USB OTG Adapter.

The Sony PHA-1, according to this Head-fi member.

The Stoner Acoustics UD120 DAC is working with the OPO, according to Head-fi's Cattlethief -- though only with UAPP.

The Vamp Verza works with stock music players as well as UAPP/UARP according to chazmann1117's findings. Update: Chazmann now reports that, "when using the settings required by the DAC for an android phone," there is "barely audible" noise. The noise goes away if you switch to "apple mode" on the Verza, but that makes the battery drain far more quickly.

The YuLong U100 works a treat with the OPO, according to mrruin.

(I recall reading that the JDS Labs C5D also works with the OPO, but I'm still looking for concrete verification.)

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History​

Most people who are interested in using external DACs with Android phones have been following DanBa's Android DAC thread on Head-fi for the past few years.

DanBa is focused on DAC compatibility with mainstream devices by Samsung and Nexus because the former has been the most popular maker of Android phones and the latter is Google's intersection with chosen hardware companies. Samsung and Nexus devices, DanBa reasons, will affect Google's implementation of USB audio most directly and so are most deserving of his time.

That's why it's unlikely he'll create a list of DACs that work with the OnePlus One.

It's also why I thought we might create one ourselves.