To set an external (global) IP address to an internal (private) IP address, you need NAT (Network Address Translation). In a regular setting where only 1 IP address is available, this would turn an internal address to a port on the external address. IE:
192.168.1.3 --> 132.68.51.43:69256 192.168.1.4 --> 132.68.51.43:57842 192.168.1.5 --> 132.68.51.43:58366
This is Dynamic NAT, as the ports are mapped to traffic automatically by the router. There is also Static NAT, where a single internal IP address is manually mapped to a single external IP address, IE:
192.168.1.3 --> 195.67.241.7 (Static IP given by ISP)
This sounds like what you're after.
The place to set this up would be wherever your NAT is done, this is usually the gateway router to the internet. You can leave your OpenBSD with its internal ip address as the address swapping will be done at the gateway router.