Can You Do Wudu with Oil in Your Hair?

It seems that the small amount of olive oil mentioned would not prevent the water from reaching the hair . 

The basic principle, as mentioned by Shaykh Ibn `Uthaymin (may Allah have mercy on him) is that if a person uses cream or oil on the parts of the body washed during Wudu, the oil either remains solid, in which case it must be removed before doing Wudu, because if the oil remains solid, then it prevents the water from reaching the skin, and in that case Wudu is not valid.  

But if the oil is not solid, rather its trace is left on the part of the body washed during Wudu, then it does not matter . But in this case, the person should make sure that he passes his hand over the part of the body in question, because usually the oil repels water, so the water may not reach all areas of the part of the body being washed. (Fatawa At-Taharah, p. 147) 

It may be added that the matter is less crucial with regard to the head because the obligation with regard to the head is to wipe it , not wash it, and wiping means that it is not essential for the water to reach every single hair. 

Shaykh Ibn `Uthaymin (may Allah have mercy on him) was also asked: 

If a woman applies henna to her hair and it dries, and the like, can she wipe over it? 

He replied: 

“If a woman has applied henna to her hair and it has dried, then she may wipe over it and there is no need for her to undo her hair and wash off this henna, because it was proven that the Prophet had applied something to his hair to keep it together and prevent it from getting dusty whilst in Ihram, and what is applied to the hair is regarded as part of it. This indicates that there is some leeway with regard to purification of the head.” (Fatawa Al-Mar’ah Al-Muslimah, 1/28)

And Allah knows best.