Oracle ORA-21561 : OID generation failed

If Oracle client connection is giving "ORA-21561 : OID generation failed" error.
Like this:


sqlplus <test_user>/<test_user_password>@<database_name>

.
.
.

ERROR:
ORA-21561 : OID generation failed

Enter user-name: <username>/<password> @ <tns connect string>


Then the problem is most likely in the client machine hosts file.
Check that there is client machine fully qualified name and short name in the client machine hosts file. If these are missing you'll get ORA-21561 errors when trying to connect server.