Oracle uses structured query language for database access.It will store data in tables.
It follows predefined schema and set up rules to govern the relationships.
MongoDB stores data as JSON format. All the related information is stored together for faster data access.
At any time, you can change the structure of records by adding new fields.
It is a database which designed for big data storage and query.
It can gain its performance mainly by key value based design and easy to scale out
To achieve the performance and scalability, MongoDB ditches the transaction support.
We cannot use for transaction related applications like Payment gateway etc.
Both Oracle and MongoDB have a rich query language.
MongoDB is not meant to replace the relational databases.It simply fullfills the different needs.