Make Oracle external authentication connections (OSA for Oracle) using a Net Service Name

1. Download the (32-bit) Oracle Instant Client from the ESRI Customer Care Portal, and unzip it to a location on the local ArcGIS Desktop machine. Change the name of the folder if desired.

EXAMPLE:
C:\OracleInstantClient_32

2. Add the Oracle Instant Client path to the beginning of the existing PATH (operating system) Environment Variable.

EXAMPLE:
PATH=C:\OracleInstantClient_32;...

3. Create a file called 'TNSNAMES.ORA' inside the Oracle Instant Client directory, and add an entry in the file specifying a desired Net Service Name, the Host machine name (with the local domain), and the Service Name.

EXAMPLE:

Code:
ORCL =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = ORASRVR.<yourDOMAINNAME>)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = ORCL)
    )
  )

4. Create a file called 'SQLNET.ORA' inside the Oracle Instant Client directory and add this entry to the file:

SQLNET.AUTHENTICATION_SERVICES = (NTS)

Note:
Setting    SQLNET.AUTHENTICATION_SERVICES = (NONE) does not suffice.

5. Create an Environment Variable on the Windows operating system called 'TNS_ADMIN' that references the location of the Oracle Instant Client directory.

6. Launch ArcCatalog and make a connection using the Net Service Name and selecting the Operating System Authentication option from the drop-down (the username and password is grayed out).