Only water adds moisture to the hair, BUT oils can seal water in the hair strand. Sealing oils should be used only on wet hair, so that they don't seal water out of the hair, but into the hair. I will point out the sealing (non-penetrating) oils in this entry with the color green.
Oils (that penetrate the hair shaft) can also replace lost essential fatty acids which are part of the molecular structure of the hair (these EFA need replacing more and more as the hair is longer) and give the hair more plasticity as well as make it feel softer. These oils can be used on dry hair, wet hair contains water inside the shaft and may keep out some of the oil. Sealing oils can keep out penetrating oils. I will point out penetrating oils in this entry with the color red.
"Oil is able to penetrate the hair shaft only if it has more affinity for the hair proteins. This affinity is consequent upon the structure of the oil. Triglycerides of principal fatty acids (like lauric acid, myristic acid, palmitic acid and stearic acid) have a relatively simple straight chain structure and low molecular weight. Therefore, these oils are able to penetrate the hair shaft better. One example of such oil is
Scientists now agree that straight chain glycerides like
"Oils are moisturizers. There are 4 types of moisturizers: Occlusives, Humectants, Emollients, and Proteins. Read more here.
Occlusives physically block water loss, and include petrolatum, lanolin,
Humectants attract water to the hair and include glycerin, propylene glycol, sorbitol, honey, syrup, aloe vera gel.
Emollients smooth the surface by filling spaces with droplets of oil and include most plant oils and butters. Some penetrate the hair shaft more than others. I made a list here of the constituent fatty acids of different oils.
Proteins replenish protein and include collagen, keratin, elastin, and protein." (source)
"Straight chain glycerides are oils that penetrate:
Oils that partially penetrate:
Polyunsaturated oils sit on top of hair:
Other interesting reads: Humectants (Moisturizing Agents), Effect of