No. 7 Bonding Oil.

"Olaplex No. 7 Bonding Oil is great because it's lightweight, it's multitasking in the sense it helps to smooth, add shine, reduce frizz, but at the same time protect hair from UV damage and heat damage, and really adds a nice high gloss finish. It's very lightweight and just applies nicely on the ends. I apply it after I shower before I blow dry my hair, and then I'll actually apply a coat in between hair washing, especially at night, to the ends. I find that it really helps to keep my hair soft and shiny and smooth in between hair washing. [...] I'm really trying to revitalize my dry, damaged hair from all the heat styling and chemical coloring I've been doing to my ends, and I find this oil is excellent because it provides the smoothing, anti frizz, glossy effect to my hair."

"This Olaplex oil, I have been using it so much every day and hardly anything's used, perfect size for travel. I've been using this every day on my hair because I've been washing my hair every day, so I'm frizzy at the moment, but this oil is really, really helping keep it under control, and it's actually a heat protectant. I've enjoyed putting this through my hair at the start of the day before we go in the sun, and then after I wash it, just to try and tame some frizz. It's a really lovely oil."