Dear Student,
Oil and water are made up of different kinds of atoms. Their properties are totally different from each other. And there is a principle which says 'Like dissolve like' , since oil and water are two unlike liquids so they do not get dissolve on mixing.
Oil is a supernatant liquid. The liquid that lies above or on the top of the sediment or precipitated insoluble material is called supernatant. It is obtained after centrifugation.
​​​​​​​Regards