RTPProxy can be run on a different machine than OpenSER.

For example, if the remote machine has an IP Address of 192.168.1.1 (which isn't the same machine that OpenSER is running on):

On the RTPProxy machine start rtpproxy with the following command:

/usr/local/bin/rtpproxy -l 192.168.1.1 -s udp:*:12345

Note1: 12345 is the port that rtpproxy will listen on. The default, if not specified is 22222.
Note2: There is a -f option that will keep rtpproxy in for foreground. This will offer you the ability to watch the execution of rtpproxy in realtime.

On the OpenSER machine add the following line to your ser.cfg

modparam("nathelper", "rtpproxy_sock", "udp:192.168.1.1:12345")

This is just a simple example. More complex configurations are possible by building on the above.