Hi Ciara ~
I use extra virgin olive oil on my hair -although any old olive oil, "extra virgin" or not, will do. I usually apply it, warm (not hot), to my damp hair for at least 15 minutes before I shampoo. In the past, I have added it to conditioners. It actually seemed to improve how Nexxus Ensure Acidifyer worked on my hair. My hair is relaxed. My mom, who has beautiful, almost brastrap length natural hair (1 inch away), mixes it with distilled water in a 1:3 ratio (e.g., 1 tbsp of EVOO to 3 tbsp of water). She keeps the combo in a spray bottle and spritzes her hair daily. Something must be right, because she started out above-shoulder length about a year-and-a-half ago. HTH
Hopefully, someone can answer your GNC Nourishair question. I've never taken them.