All necessary answers on your question can be found on
http://openvpn.net/index.php/open-source/documentation/howto.html#scope
:
1.) You must set up a route on the server-side LAN gateway to route the VPN client subnet (f.i. 10.8.0.0/24) to the OpenVPN server (this is only necessary if the OpenVPN server and the LAN gateway are different machines).
Make sure that you've enabled IP and TUN/TAP forwarding on the OpenVPN server machine.
On an OS X this is done with the following commands:
sysctl -w net.inet.ip.fw.enable=1
sysctl -w net.inet.ip.forwarding=1
natd -interface en0
ipfw add divert natd ip from any to any via en0
As you tell that you are able to access vpn server local lan from vpn client, it seems that you already did this configuration.
Next steps:
For this example, we will assume that the client LAN is using the 192.168.4.0/24 subnet, and that the VPN client is using a certificate with a common name of client2. Our goal is to set up the VPN so that any machine on the client LAN can communicate with any machine on the server LAN through the VPN.
Before setup, there are some basic prerequisites which must be followed:
The client LAN subnet (192.168.4.0/24 in our example) must not be exported to the VPN by the server or any other client sites which are using the same subnet. Every subnet which is joined to the VPN via routing must be unique.
The client must have a unique Common Name in its certificate ("client2" in our example), and the duplicate-cn flag must not be used in the OpenVPN server configuration file.
First, make sure that IP and TUN/TAP forwarding is enabled on the client machine.
Next, we will deal with the necessary configuration changes on the server side. If the server configuration file does not currently reference a client configuration directory, add one now:
client-config-dir ccd
In the above directive, ccd should be the name of a directory which has been pre-created in the default directory where the OpenVPN server daemon runs. On Linux this tends to be /etc/openvpn and on Windows it is usually \Program Files\OpenVPN\config. When a new client connects to the OpenVPN server, the daemon will check this directory for a file which matches the common name of the connecting client. If a matching file is found, it will be read and processed for additional configuration file directives to be applied to the named client.
The next step is to create a file called client2 in the ccd directory. This file should contain the line:
iroute 192.168.4.0 255.255.255.0
This will tell the OpenVPN server that the 192.168.4.0/24 subnet should be routed to client2.
Next, add the following line to the main server config file (not the ccd/client2 file):
route 192.168.4.0 255.255.255.0
Why the redundant route and iroute statements, you might ask? The reason is that route controls the routing from the kernel to the OpenVPN server (via the TUN interface) while iroute controls the routing from the OpenVPN server to the remote clients. Both are necessary.
