SQL 2005 64 bit linked to Oracle

Previously SQL 2000 DTS package on 32 bit server connected to Oracle database using Oracle Instant client and a Tnsnames.ora file. Superb - until we decided to upgrade.

Now we have SQL 2005 on a 64 bit server and Visual Studio/SSIS installed on a separate 32 bit server. With the same Instant Client and Tnsnames file on the 32 bit server, my SSIS package connects to the Oracle database as before.

The problem arises when I deploy the package to the 64 bit server. Although I had the same Instant Client setup on this server, the package could not connect to the Oracle database. After much head scratching (I've no experience with Oracle) I decided the best course of action would be to set up a linked server in SQL Server to the Oracle server and drop SSIS altogether. I've installed the Oracle 10g 64 bit client and then the 64 bit ODAC. At this point I have a new provider listed so I try to execute the following

EXEC sp_addlinkedserver 'KIL_Ora3', 'Oracle', 'OraOLEDB.Oracle', 'L500'

where L500 is the 'name' in the Tnsnames file.
This executes successfully so I try

EXEC sp_addlinkedsrvlogin 'KIL_Ora3', 'FALSE', 'user123', 'pwd123'

where the username and password are working on the other servers.
This results in

Msg 15007, Level 16, State 1, Procedure sp_addlinkedsrvlogin, Line 76
'user123' is not a valid login or you do not have permission.

I tried replacing L500 with the server ip and port but end up with the same result.

At this point I seem to be going around in circles and would really appreciate some help.

Tried with 32-bit Oracle client? I had problem with 64-bit client too, app can't talk to Oracle db. Works after switching to 32-bit client.

Thanks for your response. When I discovered the name of the database I was trying to connect to I was able to create the linked server successfully with the ip address, port and database name as follows :

EXEC sp_addlinkedserver 'KIL_Ora3', 'Oracle', 'OraOLEDB.Oracle', '1.2.3.4:1521/L500'