If the OC sent you to the CA, then that is who you will have to deal with.
You might try sending another letter to the OC and specifically request them to remove the account from collections and allow you to pay them. Not sure what your first letter said.
If the OC will not, you will have to deal with the CA.
2 options-
--continue sending PFD's until they accept or it falls off
--pay it and let it report paid, send GW letters until they accept or it falls off
What is DOFD?