There are several ways that you can do this. Usually when you connect to the openvpn server for the commercially provided services the router automatically pulls the route and dns configuration from the server. You can use the
option in the openvpn configuration to prevent this. Then you would have to setup manual routes and routing rules to direct the addresses that you want to go over the vpn. For instance:
ip route default dev tun0 table 10
ip rule add from 192.168.1.3 table 10
This means that for device that uses the ip of 192.168.1.3 to lookup routing table 10 in order to get its routing information. It would then see that the default route is through the openvpn tunnel and get routed that way. There are a few more steps that need to be done but this is the general idea. Let me know if you need more help.