There is a problem (in OS X and/or OpenVPN) which causes OpenVPN to be unable to read the default gateway when you try to connect OpenVPN through an existing PPP connection; here is a workaround:

Create a ppp start-up script /etc/ppp/ip-up and add the following:

#!/bin/sh
PATH=/sbin:/usr/sbin/:/usr/bin:/bin
gw=`ifconfig ppp0|grep inet| awk '{ print $4 }'`
route change default $gw -ifscope ppp0

Save the script and make it executable running chmod a+x /etc/ppp/ip-up.

Please note that the above script was made for interface ppp0. If for any reason you have more/other, make the changes accordingly.