1. first Verify that hosts file on both machines has 
   an entry for itself and the other machine.
2. then, make sure that the oms credentials could be
   used to login to the repository outside from oem, 
   through sqlplus or so.
else

1. Launch the Enterprise Manager Configuration Assistant. 
2. Select Edit Configuration Parameters
3. Note the service parameter specified.
4. Review the tnsnames.ora file located in the EM
   Oracle Homenetworkadmin directory.
   Verify that the service parameter noted in step 3
   exists in this tnsnames.ora file. 
   The name must be identical, including domain(if
   used). Also verify that this entry does in fact
   point to the OMS.
5. Make the necessary changes to the tnsnames.ora file.