Authenticating Database Users with Windows

Windows Native Authentication Overview
Enabling users to connect to Oracle Database without supplying a username or password

Current user database links are not supported with Windows native authentication

Windows Authentication Protocols
The Windows native authentication adapter works with Windows authentication
protocols to enable access to Oracle Database.

Kerberos is the default authentication protocol for Windows 2000.
The only Oracle Database requirement is to ensure that parameter
SQLNET.AUTHENTICATION_SERVICES contains nts in the following file on both the client and database server:

ORACLE_BASE\ORACLE_HOME\network\admin\sqlnet.ora

Operating System Authentication Enabled at Installation
When you install Oracle Database, a special Windows local group called ORA_DBA is created. Members of local group ORA_DBA automatically receive the SYSDBA privilege.

Membership in ORA_DBA enables you to

Connect to local Oracle Database servers without a password with the command
CONNECT / AS SYSDBA
Connect to remote Oracle Database servers without a password with the command
CONNECT /@net_service_name AS SYSDBA