First, examine the contents of $ORACLE_HOME/network/admin/tnsnames.ora.

  cat $ORACLE_HOME/network/admin/tnsnames.ora

If the DBCA created net service names for your instances already, then
skip the rest of this step.

To create a service name, for example for instance 'o11g':

1. Log in as 'oracle' and start netca.

     netca

2. Select "Local Net Service Name Configuration". [Next]

3. Select "Add". [Next]

4. Enter the Database Service name.
   Service Name:  o11g
   [Next]

5. Accept the default protocol, 'TCP'. [Next]

6. For the Host name, enter 'localhost'.
   Accept the standard port number, '1521'.
   [Next]

7. Select "No, do not test". [Next]

8. Accept the suggested "Net Service Name". [Next]

9. [Finish]
