When any database is created, the user SYS, identified by the password CHANGE_ON_INSTALL, is automatically created and granted the DBA role.
All of the base tables and views for the database's data dictionary are stored in the schema SYS. These base tables and views are critical for the operation of Oracle. To maintain the integrity of the data dictionary, tables in the SYS schema are manipulated only by Oracle; they should never be modified by any user or database administrator, and no one should create any tables in the schema of the user SYS. (However, you can change the storage parameters of the data dictionary settings if necessary.)
Most database users should never be able to connect using the SYS account. You can connect to the database using this account but should do so only when instructed by Oracle personnel or documentation.
When a database is created, the user SYSTEM, identified by the password MANAGER, is also automatically created and granted all system privileges for the database.
The SYSTEM username creates additional tables and views that display administrative information, and internal tables and views used by Oracle tools. Never create tables of interest to individual users in the SYSTEM schema.
The DBA Role
A predefined role, named "DBA", is automatically created with every Oracle database. This role contains all database system privileges. Therefore, it is very powerful and should only be granted to fully functional database administrators.