DCO is really just DOO with a change of name to try and avoid the controversy (and resulting industrial action) of DOO - an effort which failed miserably.
Regardless of whether or not a 2nd member of staff is "planned" to be on all services, which may be "subject to change in times of disruption" or whatever other ways out that TOCs / the DfT try and come out with, the situation is quite simple.
If a train can run under any circumstances, in public service, with just a Driver on - then it's DOO. If the lack of a 2nd member of staff means that that service must be cancelled, then that service is not DOO.
So to give some examples:
ScotRail - on most Strathclyde electric services, a ticket examiner is rostered to every service. But if for whatever reason they're not on board, the train can still run. This is DOO, or DCO if you prefer.
On those services that were recently electrified (E-G, Stirling etc) a guard is rostered for every service. If for whatever reason they're not on board, the train is cancelled. This is not DOO, or DCO - this is DGO. The fact that in this particular circumstance the guards only close the doors, with the driver opening, is irrelevant. The train can't run in public service without a guard, therefore it's not DOO.