https://openwrt.org/docs/guide-user/firewall/firewall_configuration#zones
The masq option handles NAT. Make sure you assign vpn interface to a different zone than the wan, otherwise removing NAT from wan zone might break your internet connection for the hosts.
You can also route the /24 to an interface. No NAT needed.
Though it doesn't need to be a /24, and you don't need to route it to an interface. You could route a single address (/32) to the private IP address of the client. But in this case you need to configure the preferred source address (src) in the routes on the client. Otherwise it will use the private IP address as source address by default. The OpenWrt router also need to use policy routing which sends traffic from the client to the internet via the (openvpn) tunnel.
I suggested routing, so the OP didn't have to configure anything further but DHCP; but I see your point!
Which is the easiest way to do that?
I need to do it very fast, I'm going to add a couple of servers in the network
Can you make me an example of DNAT and SNAT on the OpenVPN server?
What is the OS of the OpenVPN server?
Debian 9 with iptables and shorewall
https://www.frozentux.net/iptables-tutorial/iptables-tutorial.html#DNATTARGET
https://www.frozentux.net/iptables-tutorial/iptables-tutorial.html#SNATTARGET
SNAT: iptables -t nat -A POSTROUTING -o tun0 -s Client2_IP -j SNAT --to Public_IP
DNAT: iptables -t nat -A OUTPUT -d Public_IP -j DNAT --to Client2_IP
How can I easy forward the Client_2 traffic on the tunnel?
Better like this
This act like a DMZ? (If I have a web server on Client2 it will be accessible either on the public IP?)
Yes every packet destined to Public_IP is forwarded to Client2_IP.
Thanks, I'll try ASAP, how can I forward the traffic from OpenWRT?