Sodium is kept in kerosene oil because of its highly reactive property with the atmosphere .Sodium can catch fire if it is kept in open air so it is kept in kerosene to prevent from catching fire

Being sodium a very reactive metal, By reacting with water or air its get explode nature. In order to avoid this, It is stored in kerosine as just like all the 1A group elements store except lithuim in paraffin wax.
1 5 1