There are two processes for contesting the debt.
One is by way of dispute of its accuracy under the FCRA, and the other is to request debt verification with the debt collector under the FDCPA.
Since you have already disputed under the FCRA and they have verifified, that option is concluded unless you can support a new dispute.
Verification of a dispute does not require that they provide documentation, or "prove" its legitimacy. They are required only to conduct a reasonable investigation, and make a determination based on that investigation. Since the OC is most likely the basis for their investigation, and the OC has also told you that they agree, it appears there would be little chance that the debt collector did not conduct a reasonable investigation.
You can file a second dispute provided it is not substantailly the same as your prior dispute. A second dispute is not substantially the same if it presents new information in support of the dispute that was not previously submitted. If you have no additional documentation to support a new investigation, they can dismiss any renewed diespute as "frivolous or irrelevant."
The second process is to send a request for debt validation to the debt collection under the provisions of FDCPA 609(b).
On the plus side, a DV request does not require any documenatation in support of the request.
On the negative side, it does not compel a response on their part. A DV request, if timely, imposes a cease collection bar on them until such time as they might choose to provide verification. If your DV is not timely, meaing not sent within 39 days of their dunning notice, it does not invoke a cease collection bar, and they can ignore it and proceed with their collection activities.
As for what you might hope to obtain with a DV, it also does not require them to provide any supporting documenation. They must simply obtain verification, and communicate that finding to you. With the statement from the OC that they concur, it would appear that verification of the debt by the debt collector would be pro-forma.
To contest the amount of the debt would most likely now require that you bring civil action, forcing them to provide supporting documentation of the amounts making up the debt. Neither the dispute or DV processes entitle the consumer to documentation.
