How do the oil immersion objective differ from the low to high power objective?

Answer 1

low power objective or LPO has a magnification of 10X.
high power objectiv or HPO has a magnification of 40X.
Oil immersion objective or OIO has a magnification of 100X.
with LPO and HPO, air is between the objective and the sample. while in OIO, an immersion oil is put between the objective and the sample. thus, the reflected light does not scatter but concentrated to the objective, making the sample you observe clear at that kind of magnification.