In the /etc/openvpn/server.conf file on the server you can set routes for both ends that will be added when the tunnel is created. For example to add a route on the client for the server's network:
push "route 192.168.178.0 255.255.255.0"
For the server to get to the client network:
route 10.8.0.0 255.255.0.0
For any other systems on either network to connect to each other, you need to enable forwarding in the client and server,and to have routes set up at each end that point to the machines with the tunnel endpoints. If the openvpn server is already the default gateway (the usual case), then nothing else is needed.