I have this configuration in my network.
The DSL modem is connected to internet with static IP and my OpenWrt router with OpenVPN server is connected to DSL modem.
How should I configure DSL modem and OpenWrt router to access my Server and clients via Internet? What should be the configuration for OpenWrt?
I probably would configure the modem in bridge-mode (or replace the modem with one that can be configured into bridge mode), and put the server behind the OpenWrt router and the use port forwards and access rules (both in the router's firewall settings) to allow access to selected ports on the server from the outside.
Currently your "modem" also acts as router, you probably could also configure your desired port forwards in the modem-router's GUI to at east make some of the server's ports reachable from the internet, and the same would work for reaching the clients except you need to coordinate port forward and access rules in both the modem-router and the OpenWrt router (if that is actually operating as router, from your IP addresses it looks like it is not acting as true router).