Operating System Authentication Enabled at Installation

When you install Oracle Database, a special Windows local group called ORA_DBA is created (if it does not already exist from an earlier Oracle Database installation), and your Windows username is automatically added to it. 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.

1. CONNECT / AS SYSDBA

Connect to remote Oracle Database servers without a password with the command

1. CONNECT /@net_service_name AS SYSDBA where net_service_name is the net service name of the remote Oracle Database server

Perform database administration procedures such as starting and shutting down local databases
Add additional Windows users to ORA_DBA, enabling them to have the SYSDBA privilege.