SYS and SYSTEM DBA role

SYS
When any database is created, the user SYS 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 user SYS. (However, you can change the storage parameters of the data dictionary settings if necessary.)

Ensure that most database users are never able to connect using the SYS account.

SYSTEM
When a database is created, the user SYSTEM is also automatically created and granted the DBA role.

The SYSTEM username is used to create additional tables and views that display administrative information, and internal tables and views used by various Oracle options and tools. Never create in the SYSTEM schema tables of interest to individual users.

The DBA Role
A predefined role, named DBA, is automatically created with every Oracle database. This role contains most database system privileges. Therefore, it is very powerful and should be granted only to fully functional database administrators.

The DBA role does not include the SYSDBA or SYSOPER system privileges. These are special administrative privileges that allow an administrator to perform basic database administration tasks, such as creating the database and instance startup and shutdown