You can do this in the OpenVPN setup with "Locale Networks" and "Remote Networks" on server and client site. How exactly, depends on which site is the server and which is the client.
Just enter the one host you want to access.
But if you have also set a route form IPSec for the same destination, I don't know which one has the higher priority.
However, you may also be able to direct the traffic to OpenVPN by policy based routing, after assigning an interface to the VPN instance.
If you just want get access from site A to B, the simplest solution would be to do NAT for this connection at B (by adding an outbound NAT rule).