While creating an OpenVPN server you should think what you are going to use it for.
Long story short
Use tap for a layer 2 vpn, tun for a layer 3.
Tap is a virtual eth interface and, you will be able to put the interface
tap0 into a bridge:
apt-get install bridge-utils brctl addbr br0 brctl addif br0 tap0 eth0
Tun is for a direct IP connection between two points.