I have done back-to-backs in the past, just got lazy lately. I had read that OP can help with grey hair so I stopped dying mine altogether several months ago. I have my natural colour back for the first time in over 20 years. I do have some grey and it's pretty noticeable to me, but others say it's not noticeable unless you look hard. It's very evenly spread throughout so I think that makes it look more natural. Because of the hair loss and delicate condition of my hair I don't intend to go back to harsh chemicals for colouring. I am hoping if I OP enough it will help with the colour too.
My favourite used to be VCO, but mostly I eat that now and my fav OP oil is macadamia. They grow macadamia's not far from where I live, so I am working on the theory that the oil I use is really fresh (probably just kidding myself), but hey, I like the taste.