Sayyed Muhammad Husaini Ragheb, Sayyed Muhammad Husaini Ragheb has a BA in Law from Guilan University, Iran and has also undertaken Hawzah studies in Qom. He is a Cultural Affairs director of Ethics Group of Al-Mustafa Open... Answered 2 years ago

The oil prevents water from reaching the hair or the skin
So one should make sure that the water comes into direct contact with their hair or skin.
You can leave a little area of the head without oil in order to do a correct wudhu.