Place a small drop of oil on the objective lens. A small drop of oil must also be placed on the cover slip. The lens should be carefully lowered into the oil prior to focusing. Observe this with your naked eyes focusing on the objective and the specimen. Do NOT look through the oculars.
Oil improves the optics because it unites the glass cover slip and the objective.
It replaces air with oil. The oil has the same refractive index as glass. Be sure that the specimen was in focus at 40X before switching to 100X. Avoid focusing down on the specimen with an oil immersion lens. Change the focus so that the objective is traveling away from the slide. If the image does not come into focus, reverse the direction until it does.