I was having the same problems because I had never attempted to configure everything before. My main error was that under host/service I kept using the SID or SERVICE name of my database/instance.

The answer for me was that you need to use the actual name of the server or IP of the server in order to connect thru the OMS. Consider that the OMS is located on the MACHINE so you are connecting to the MACHINE (IP address) and not the database/instance.

Try that and let me know what you get.

- Magnus