OPC works seamlessly through most routers. However, any router that provides NAT (Network Address Translation) services will stop DCOM from working. This is because the OPC Server PC must be able to address the OPC Client PC directly.
In practice, most routers that are internal to a site/company do not use NAT. However, Routers that provide communication to the Internet typically use NAT and will stop OPC communication. If your OPC applications are separated by a router that uses NAT, you will have to either use Tunneling technology or use OPC UA (which uses web-services to establish communication instead of DCOM).