is caused by a fungus called Malazessia. This fungus is a normal commensal( residing in the body). Some people will mount an allergy
against it causing dandruff. This fungus lives in oily skin. You seem to have an oily skin, that is why you suffer from acne as well.
Use a shampoo containing Ketoconazole
2% and ZPTO 1% thrice a week for a month. After that continue using one a week at least for maintaining the improvement. Do not apply oil on your scalp.
If you have tried this treatment as well earlier and still it has not come down you need to take a dermatologist's consultation. A reassessment of diagnosis is required to rule out seborrhoeic dermatitis
and scalp psoriasis
as both of them can present with resistant scaling over scalp which resembles dandruff. The treatment in these cases will be slightly different.
Hope this helped