SQL Error: ORA-28500: connection from ORACLE to a non-Oracle system returned this message:
[Microsoft][ODBC Driver Manager] Data source name not found and no default driver specified {IM002}
ORA-02063: preceding 2 lines from MSSQL
28500. 00000 -  "connection from ORACLE to a non-Oracle system returned this message:"
*Cause:    The cause is explained in the forwarded message.
*Action:   See the non-Oracle system's documentation of the forwarded
           message.

Listner.ora

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PLSExtProc)
      (ORACLE_HOME = C:\oraclexe\app\oracle\product\11.2.0\server)
      (PROGRAM = extproc)
    )
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = C:\oraclexe\app\oracle\product\11.2.0\server)
      (PROGRAM = extproc)
    )
    (SID_DESC =
      (SID_NAME = MSSQL)
      (ORACLE_HOME = C:\oraclexe\app\oracle\product\11.2.0\server)
      (PROGRAM = dg4odbc)
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.5)(PORT = 1521))
    )
  )

DEFAULT_SERVICE_LISTENER = (XE)

TNSNAMES.ORA

XE =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.5)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = XE)
    )
  )

EXTPROC_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
    )
    (CONNECT_DATA =
      (SID = PLSExtProc)
      (PRESENTATION = RO)
    )
  )

ORACLR_CONNECTION_DATA = 
  (DESCRIPTION = 
    (ADDRESS_LIST = 
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1)) 
    ) 
    (CONNECT_DATA = 
      (SID = CLRExtProc) 
      (PRESENTATION = RO) 
    ) 
  ) 
MSSQL =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.5)(PORT=1521))
    )
    (CONNECT_DATA =
      (SID = MSSQL)
    )
  (HS = OK)
)

SQLNET.ORA

# This file is actually generated by netca. But if customers choose to 
# install "Software Only", this file wont exist and without the native 
# authentication, they will not be able to connect to the database on NT.

SQLNET.AUTHENTICATION_SERVICES = (NTS)

initMSSQL.ora

# This is a sample agent init file that contains the HS parameters that are
# needed for the Database Gateway for ODBC

#
# HS init parameters
#
HS_FDS_CONNECT_INFO = MSSQL
HS_FDS_TRACE_LEVEL = off


#
# Environment variables required for the non-Oracle system
#
#set <envvar>=<value>

ODBC Data sourcerce Name in ORACLE Server is:  MSSQL

DBLINK

CREATE public DATABASE LINK MSSQL CONNECT TO mssql_user IDENTIFIED BY mssql_pwd USING 'MSSQL';