If someone has oil on his or her head and the person enters in a state of janabah does the person have to wash his or her hair with shampoo for ghusl to let the water touch the hair?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If there is some oil or any other thing that prevents water from reaching the roots of the hair, it should be removed by shampoo or any other detergent.

In fact, water must touch the skin of the head in Janaabah Ghusl (ceremonial bath) as washing the whole head is an obligation in Ghusl.

Allah knows best.