If your OVPN setup uses tap devices, the solution is easy since you can bridge tap devices.
If you're using tun you can't, so the pi would need to route.
You cannot mix tap and tun devices: all clients and server must use the same type.
Step 1 Make the Pi work as an openvpn client.
Step 2 Get a secondary USB ethernet interface.
Step 3 Add a bridge (with members usb0 + tap0) or a dedicated network (static address on usb0 + dnsmasq for clients) and filter/route this to eth0
For step 3 you can have a look at a routed or bridged AP configuration for Pi. What you require is simply a variation of that.
"S'il n'y a pas de solution, c'est qu'il n'y a pas de problème." Les Shadoks, J. Rouxel