here is two simple command to have openvpn tunnel up between server and client on linux system
on server :
/usr/local/sbin/openvpn --port PORT --proto udp --dev tun0 --bcast-buffers 512 --fast-io --ifconfig 192.168.20.1 192.168.20.2 --comp-lzo --comp-noadapt --daemon --route some_net its_masking 192.168.20.2 --keepalive 30 90
on client :
/usr/local/sbin/openvpn --remote SERVERADDR --port PORT --proto udp --dev tun0 --bcast-buffers 512 --fast-io --comp-lzo --comp-noadapt --daemon --ifconfig 192.168.20.2 192.168.20.1 --keepalive 30 90 --float