I've searched the whole internet and still didn't find a solution... or maybe I did and overlooked it . Ok... the problem is this:
- i set up an Asus WL500gP with OpenWRT
- installed OpenVPN on it to use it as a client to connect to the office
- managed to configure it and now it connects and can access the office lan from my router
- what do i have to do to make the computers from behind the router to see the office lan?

   In my limited knowledge I'm guessing it's a routing problem and that I have to set the router to allow traffic between the lan interface (br-lan) and the vpn interface (tun). I've got no i idea how to do this... can anyone please help?

couple of things you need to do (some I guess you have done already)

1) turn on ip forwarding otherwise packets will not be routed through the machine, but I am guess this is already on
2) is your openwrt box the default gateway for your network
3) fix up iptables
   a quick and simple solution is

iptables -I FORWARD -i <internal interface> -o <openvpn interface> -j ACCEPT

Which basically says allow through anything that is coming from your local lan and going out the openvpn interface

or you could do it by ip address

iptables -I FORWARD -s <source network> -d <destination network> -j ACCEPT

Alex

Thanks for the reply! I'll give it a shot as soon as i get home... now I'm at work and there's no way to test if it's working . Anyway, I think the first option is better because the lan at work has more than one ip class. So the iptables should look like this:
   iptables -I FORWARD -i br-lan -o tun0 -j ACCEPT  ?

That didn't work

After some more searching and requesting help, I managed with this:

iptables -t nat -I POSTROUTING  -o tun0 -s <local ip class> -d <ip class behind vpn server> -j SNAT --to <router ip on vpn interface>
iptables  -I FORWARD -s <local ip class> -d <ip class behind vpn server>  -j ACCEPT

Something like this:

iptables -t nat -I POSTROUTING  -o tun0 -s 192.168.1.0/24 -d 192.168.2.0/24 -j SNAT --to 10.4.1.2
iptables  -I FORWARD -s 192.168.1.0/24 -d 192.168.2.0/24  -j ACCEPT

Maybe this'll help somebody else too... good luck!