Oracle Default Users

When you are creating Database , By Default there’s Users Created In Installation Phase , Check Them :

The SYS user owns all base tables and user-accessable view of the data dictionary (Oracle configuration information). No Oracle user should ever alter (update, delete, or insert) any rows or schema objects conatained in the SYS schema, because such activity can compromise data integrety. The security administrator must keep strict control of this central account. 
The SYSTEM user is used to create additional tables and views that display administrative information, and internal tables and views used by various Oracle options and tools. 
The SYSMAN user represents the Enterprise Manager super admin account. This EM admin can create and modify other EM admin accounts as well as admin the database instance itself.
The DBSNMP user is used by EM to monitor the database. EM uses this account to access performance stats about the database. The DBSNMP credentials sometimes referred to as the monitoring credentials. 
In addition to these users, a user can connect with diffrent levels of privileges, namely SYSDBA and SYSOPER. When you connect using “connect sys/passwd as sysdba” your connecting as the SYS user and requesting SYSDBA privs. Because the SYS user is the Oracle equivilent to the UNIX root user Oracle makes you specify the amount of control you have, which is why you’ll get an error if you try to connect without specifying the privs.
The big diffrence between SYSDBA and SYSOPER privs is that SYSDBA can do anything (just like root). The SYSOPER privs allow you just about the same amount of control but won’t allow you to look at user data. Both privs allow you to ALTER DATABASE, CREATE SPFILE, STARTUP or SHUTDOWN, ALTER DATABASE ARCHIVELOG, and includes RESTRICTED SESSION privs. However, only SYSDBA can CREATE or DROP DATABASE, and the ALTER DATABASE RECOVER options for SYSOPER are limited to complete recovery only. 
There’s Another Users you can find Them On This Link 
Thank you 
Osama mustafa

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