DOO-P stands for Driver Only Operation - Passenger, in technical terms a DOO train is a non passenger train, empty coaching stock, engineers or a freight train etc.
DCO was a term coined at the start of the Southern Dispute a few years ago now, I'm not sure of the exact origin of the term but I'm sure someone came up with it during a tea and biscuits meeting between the DFT/ORR, RSSB and the TOCs...
In, short it's a sop to make DOO seem more palatable to both the public and the workforce. From a strictly technical and legislation point of view a DCO train is a DOO-P train.