Chris,
There is no standard in OSHA covering the length of hair. If you were to get cited it would be under the general duty clause. OSHA gives employers the responsibility of creating a safe work place irregardless of whether there is a rule in place.
I suspect you have a work place where machinery is spinning at a high rate of speed and you're worried about someone's hair getting caught. At many papermills, employees and visitors are not allowed to wear ties near the paper machine. The paper travels up to 30 miles an hour and can literally suck the tie right in.
In my opinion, the company is well within its rights to require employees with long hair to wear a hairnet or put their inside a lab coat.
MikeCJ