I experienced the same problem and anti-dandruff shampoos did not help me either. I think it's because it's not truly dandruff but dry skin. Here's what worked for me:
1. I switched to a sulfate-free shampoo, these do not have the harsh detergents that most shampoos have in them and they are more gentle on your hair and scalp. I used DermOrganics for awhile (available on amazon.com) and I used L'Oreal's Intense Nourishing Shampoo for awhile, both were good. The L'Oreal shampoo is available at your drugstore and is very reasonable, DermOrganics costs a bit more.
2. I used an intensive moisturizing conditioner. I tried several brands, tried some hot oil treatments, etc and the product that worked for me was L'Oreal Power Moisture Moisture Rush Masque. It comes in a teal jar, I would put quite a bit of it on, let it absorb while I bathed, shaved, etc and then rinse it out. Sometimes I would also add drops of jojoba oil for added moisture but the conditioner itself seemed to be enough to stop the flaking.
3. I stopped washing my hair as often. I went from washing my hair daily to every other day, occasionally every 3 days. Like you, I normally had oily hair so that was a big change. Now that I have been off Accutane for three months, my hair is now oily again.
When I went to my salon I would also request a moisturizing treatment which was helpful but not absolutely necessary, just something else to consider. =)
Good luck to you!