Does the tunnel endpoint have a route to LAN via OPNsense?If there isn't, you'll have to do outbound NAT for OpenVPN in OPNsense
Check firewall rules on the WAN interface accordingly. I guess that by "NAT Rule"you mean port forward, right?
I checkek on the Outboud NAT, and there is no automatic rule generated. On the Outbound NAT, there are 4 options and the current is automatic NAT generation.So, did you mean I have to add manual outbound NAT rules ?