On Mac’s OS X it seems at this time that OpenVPN does not tunnel IPv6, so if your on a network using IPv6 then your VPN is essentially not working.
The only work around I can find is to turn off IPv6 and simply use IPv4:
networksetup -setv6off Wi-Fi
networksetup -setv6off Ethernet
To restore IPv6:
networksetup -setv6automatic Wi-Fi
networksetup -setv6automatic Ethernet