If you’ve dealt with hair breakage, slow growth, or thinning strands, you’ve probably tried products that promise rapid hair growth. I asked five experts for the lowdown on the key ingredients and formulas to look for in the right hair growth oil for different scalp needs and hair types. Rosemary oil increases blood flow to hair follicles, oxygen, and nutrients promoting hair growth, dermatologist Dr. Ramya Garlapati, MD, recently told us.