The function of an oil immersion objective for a microscope is to achieve greater clarity of an image at high magnification. Oil prevents light from bending and distorting the image of an object under study.
The function of a microscope to produce a magnified image of an object depends on the physical properties of light. Various mediums, through which light passes, have different refractive indexes. For microscope work, the mediums are usually glass and air. Traveling from one to another, light bends and the image becomes less clear. This is the reason why it is hard for the microscope to magnify beyond 400 times without immersion techniques.
Immersion oil is designed specifically to have the same refractive index as glass. As a result, there is no bending of light in between the specimen and the microscope's objective. Thus, magnification of 1,000 times and more becomes possible.
To use an oil immersion objective, it first needs to be focused on the object with a low magnification. Then the specimen is covered with a drop of oil. Next, the oil immersion objective is lowered directly into the oil. Care needs to be taken not to damage the specimen at this point. Finally, magnification can be increased to receive a more detailed image of the studied object.