A short definition of an RDBMS is: a DBMS in which data is stored in tables and the relationships among the data are also stored in tables. The data can be accessed or reassembled in many different ways without having to change the table forms.
according to my opinion based on security ORACLE is far better than others..
Oracle is more stable, more robust, and more secure
Oracle is premium soft, it is best for any system setup relational database, many supporting tool , but no free.
Oracle is more geared towards and thus may perform better with very large deployments, it is true, but the thing that actually makes a difference is having optimized queries and sensible indices.
It really depends on what you want to use it for. Oracle is better for larger sites (it's better at handling a lot of simultaneous requests at once).

MYSQL I'd say is easier to use and there are a lot more resources out there for it than Oracle.