Does olive oil on the hair prevent water from reaching it, thereby invalidating ablution?

A small amount of oil does not prevent water from reaching the hair. The rule is that if the grease (cream and oil) has a substance that prevents water from reaching the skin, it must be removed before purification. However, if the grease has no substance, then it does not matter, and in this case, it is confirmed to pass the hand over the limb. In addition to that, wiping the head involves leniency, as its obligation is wiping, not washing, and it is not necessary for water to pass over every single hair. It is also permissible for a woman to wipe over her head if she has matted it with henna or the like, because matting is considered part of the head, and this indicates the ease in purifying the head.