This is assuming that the debt has been formally assigned to the DCA and not that they are collecting on behalf of the OC.
It doesn't matter Jack Shout what the DCA tell you about ownership - this needs to be a properly drafted Notice of Assignment sent to you by the OC. For the Assignment to be absolute it needs to be sent by recorded post or hand delivered. If only sent by snail mail then the DCA will hold Equitable Assignment and be entitled to collect the debt, but the right to sue you for it will not transfer and only the OC could do that. Keep details of any delivery including envelopes as these can come in very handy later.