1)

SQL> show parameter os_a

NAME                                 TYPE        VALUE
------------------------------------ ----------- --------------
os_authent_prefix                    string      OPS$
remote_os_authent                    boolean     FALSE


2) sqlnet.ora has following setting

SQLNET.AUTHENTICATION_SERVICES = (NTS)

3) I have created user like following

sql> create user OPS$CLASS
  2    identified externally
  3    default tablespace user_data
  4    temporary tablespace temporary_data
  5    profile clas_prof
  6    account unlock;

user created.

sql>   grant connect to OPS$CLASS;

Grant succeeded.

C:\Documents and Settings\CLASS_user>sqlplus

SQL*Plus: Release 9.2.0.1.0 - Production on Wed Apr 20 10:18:58 2011

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Enter user-name: /@DB01

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.6.0 - Production

SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.6.0 - Production

C:\Documents and Settings\CLASS_user>sqlplus

SQL*Plus: Release 9.2.0.1.0 - Production on Wed Apr 20 10:19:29 2011

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Enter user-name: /@DB02
ERROR:
ORA-01017: invalid username/password; logon denied