To connect to the wide-spread legacy OPC servers and clients an intermediate software is required. High speed and robust implementation give the full advantage of secure communication and any DCOM headaches can be avoided. You can actually use wrapper and proxy to tunnel COM based legacy OPC via secure and firewall friendly OPC UA.
A wrapper is required to connect a COM based legacy OPC client (e.g. an existing HMI) to any OPC UA server that supports the DA-Profile.
A Proxy is needed when connecting a new UA enabled client to a COM based legacy OPC Data Access server, e.g. any DA2 or DA3 device server.