I've used both. I am natural and what I found about Nioxin is that it is more geared towards a scalp treatment. When I went to the salon, my stylist would always use Nioxin on me and after the Nioxin conditioner, she would follow it with a moisturizing conditioner. OCT seems to be geared more towards the hair and serves as a scalp and mositurizing treatment.
Disclaimer: I'm definitely not an expert, but just tried both for extended periods of time. HTH.