How can i keep my hair straight?

I have very thick hair & its a few inches shorter then shoulder length Also I have an oily scalp. I have several shampoos for this, after i dry my hair my hair becomes a mess, a huge deflated afro although my hair isn't very curly, except near the ends. I really want to keep my hair straight for as long as i can. I've been using gel but it makes it hard & irritates my scalp. Any suggestions, tips or tricks?