OpenVPN restriction
Solution 1:
I don't believe OpenVPN has any built-in method to control access based on IP address. You probably will need to use either your border firewall, or the firewall for that host OS to limit access as required.
Personally, I am not sure that it is worth the effort. If you have a good set of keys and HMAC authentication there is very low chance of anyone else being able to get it.