Hello,
There is no medical condition as such which causes oiliness of the hair. I would suggest you to clean your hair frequently with a mild shampoo (you can use the ones for oily hair) and don’t apply any conditioner on the hair.
It makes sense to avoid the use of conditioner as it also tends to make the scalp oily. When shampooing the hair, switch from warm water to cool water after you've thoroughly rinsed away the shampoo. The cool water rinse helps close the pores of the scalp and slow down the oil production. Please do consult a dermatologist and get it evaluated.
I hope it helps. Take care and regards.
Maybe try Nizoral shampoo. It's available in stores, but it's not cheap. Wet your hair, apply the shampoo and leave it on for about 3 to 5 minutes, then rinse and repeat again.
Hope this helps.