I have a backup server I would like to use off site, and connect it to the site using OpenVPN. It would be behind a NAT firewall. Would I just need to give it an address on the same subnet as the rest of my on site network, and then forward UDP 1194 to it? Or is the configuration more complex than that?
If the remote server is going to act as the OpenVPN server then it will need to receive inbound traffic from the Internet (with the default for UDP being port 1194, as you stated). If it's going to act as the client and initiate the connection to another OpenVPN server then it only needs to be able to source traffic to the server and receive responses back (which any modern stateful firewall will handle fine).