I'm sitting here right now with rosemary oil in my hair 
It's only my second time applying it so can't given any anecdotal evidence as to whether it's effective or not.
This article is interesting
www.womenshealthmag.com/beauty/a41902382/rosemary-oil-for-hair/
“Rosemary has been shown to have significant antimicrobial, anti-inflammatory, anti-oxidant, anti-apoptotic, anti-tumorigenic, antinociceptive, and neuroprotective properties,”
"In a 1998 randomized, controlled, double-blind study of 86 patients with alopecia areata, it was found that an aromatherapy essential oil blend of thyme, rosemary, lavender and cedarwood was way more effective than the placebo. Basically, rosemary oil has been found to be equally as effective as topical minoxidil, which treats androgenic alopecia, but it’s safe for all hair loss patients."
"Rosemary oil helps hair grow by “blocking the effects of DHT, which is a type of testosterone that is normally responsible for shrinking hair follicles that can lead to hair loss,”