Try hair powder. The powder soaks up the grease. It sounds weird, but it really works. Plain ol' cornstarch or baby powder works, but if you have dark hair, it can look weird. Bumble & Bumble makes a great hair powder you can use that even comes in 4 shades. It's just called Bumble & Bumble Hair Powder.
I've also heard that using oil blotting sheets (the kind used for a greasy face) work on oily hair. I know Clinique and Clean & Clear make them. Also check out "dry shampoos". One of my friends is OBSESSED.
Putting your hair in ponytail, wearing a bandana, or...yes, a cute hat or visor are easy ways to deal with it. And don't worry, most people will be in your shoes. No one expects perfection on a camping trip. Good luck!