I was playing with DxO today... one thing I couldn't figure out is how to reset a photo back to the original RAW (undo all adjustments)... How do you do that?
In the Customize tab under "PRESET EDITOR" there is a Preset called "4 - No correction". Apply that preset and you're back to your RAW file. If you want to revert to the default starting point (assuming that you haven't modified your default preset) apply the "1 - DxO Standard" preset.
Also, although there is no history tab like in Lightroom, you can step back one adjustment at a time using the Undo function (Command-Z).