Oil should not be used until after cleaning your shoes.  To clean your shoes, use a damp cloth and lightly wipe away any grime or grit attached to the surface.  Wipe them again with a dry cloth to ensure they are dry and ready to get oiled. Applying oils to your leather shoes will restore and nourish them, leading to softer shoes.  There are many oils which can soften your leather shoes and prevent or reduce stiffness.  Mink oil, saddle soap, coconut oil, and foot oil are all useful for softening leather shoes.  Seek the aid of a qualified shoe repair professional to help you decide which kind of oil might be best for your shoes. Olive oil has many adherents, but some report that it leaves oily spots behind and leads to deterioration of the leather. Beware of leather oils which contain pine tar, castor oil, mineral oils, and other toxic chemicals.  These ingredients might harm or cause the decomposition of your leather shoe.  Carefully read the label before purchasing a particular leather oil. Consult the sales staff from whom you purchased your leather shoes as to which oil is best for your shoes.

TL;DR in Vietnamese:
Lau sạch giày trước khi thoa bất cứ thứ gì lên bề mặt giày. Chọn dầu. Đọc nhãn sản phẩm dầu dành cho da thuộc.