Chronic Dry Hair

I've been having this problem for quite a while now and have never really been able to fix it. Whenever I shampoo or condition my hair, no matter what I use, my hair becomes extremely dry. I've tried not shampooing/conditioning my hair so that it has a chance to build up some oil (I think Roaccutane killed most of my hair's oil production) but I get an itchy scalp within a week, whereas it takes about 3 weeks for my hair to get its perfect amount of oil where my hair is nice and managable. So I'm forced to either have rediculously dry hair (it looks like straw) and a normal scalp, or nice hair and a damn itchy scalp.

Essentially what I need to do is get rid of the itchyness for good and then let my hair do its thing naturally and build up its own oil (sebum?). I've tried using anti-dandruff shampoo but the itchyness keeps on coming back and I'm beginning to believe that these anti-dandruff shampoos are a load of bulls**t. So as you can see I'm in a bit of a catch 22 moment right now with no way to get out of it.

If anyone has any advice please let me know, I'm heading back to university next month some time and would really be happy if this problem could be solved before then. Thanks, cheers.