Here is how you set a static IP for a OpenVPN client setup through your pfSense router. We needed this for printing from one network to the other, when using the OpenVPN software client.
It might depend on how you setup your OpenVPN in the beginning, but this is what worked for me.
We have each client setup with their own username and password under System > User Manager
How to set the static IP is you go to VPN > OpenVPN > Client Specific Overrides
Click the green plus
You only need to do a few things to get this to work.
Server List Select your server from the list, I only had one.
Common Name Enter the Username of the person that you created in the User Manager.
Advanced ifconfig-push 188.8.131.52 255.255.255.0;
Type that info into the Advanced field, make sure you use an IP address within the range of the OpenVPN. Not the local IP for the PC. (It should have a local IP and the OpenVPN IP.
You will need to disconnect and reconnect the OpenVPN client if its already connected
See picture below