An oil immersion objective requires you to place a drop of immersion oil on the slide and then to rotate the 100x oil objective into the oil. The bottom part of the objective is then covered in oil and will require cleaning after the observation session. This way there is no air between the slide and the objective and this increases both brightness and resolution. It goes without saying that the immersion oil will make the slide dirty (and possibly also the other non-oil objectives, if not carefully used). For this reason a 100x oil immersion objective is not recommended when using primarily commercially made slides (which you want to keep clean and oil free).
Considering the relatively high cost of oil immersion objectives and low range of applications, I would rather save the money or invest it in a more stable microscope body. Alternatively you might consider to buy a 20x objective to have in intermediate magnification between 10x and 40x, or a 60x objective. The oil immersion objective is useful if you already know that you will be observing mostly heat-fixed bacteria, or sub-cellular structures, such as the chromosomes of dividing cells.