Should I put this oil on my hair lengths or just the scalp?

This is primarily a scalp oil. Apply it along your partings and massage it into the roots, because the purpose is to soften buildup and condition the scalp surface before washing. You do not need to apply oil directly to the middle or ends of your hair.

If your lengths look dull and you want a softer look, you can smooth the leftover oil from your palms onto the mid-lengths and ends after applying it to the scalp. That is enough — the product is washed out, so any effect on the lengths is light and temporary. Saturating long hair with the oil will only make shampooing more difficult and will not add value for flakes at the roots.

For very dry ends, you can gently work a small extra amount into the last few centimetres with your hands. This is optional and should be adjusted to your own hair; the formula is not designed as a heavy leave-in conditioner, so keep the amount small.