What's the default password for SYSTEM in Amazon Oracle RDS?
The SYS user, SYSTEM user, and other administrative accounts are locked and cannot be used
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The Amazon documentation for Oracle on Amazon RDS says you have to use master user which has the DBA privilege and other users like SYS and SYSTEM are locked by default and cannot be used. 

SYSDBA privilege is not available for customer in Amazon RDS so Amazon EC2 might be a better choice for you and can have full control over the database and high privileged user like SYS and SYSTEM user.

Useful links:
Best Practices for Running Oracle Database on Amazon Web Services
Common DBA Tasks for Oracle DB Instances
Oracle on Amazon RDS