The barrier is a wall, not a film
The outermost layer of skin behaves like brickwork: flattened cells are the bricks, and a lipid matrix of ceramides, cholesterol and free fatty acids is the mortar. When the mortar thins, water escapes faster and irritants enter more easily. Almost every symptom of 'dry skin' traces back to that mortar.
This is why simply adding water to skin does little on its own. Water applied without anything to hold it evaporates within minutes, sometimes leaving the skin drier than before.
Not all plant oils behave the same way
Plant oils differ mainly in fatty-acid profile. Oils high in linoleic acid tend to be light, absorb quickly and are associated with barrier-supportive behaviour. Oils dominated by oleic acid feel richer and more occlusive, but in some studies high-oleic oils have been linked to increased barrier disruption when used on already-compromised skin.
Black seed (Nigella sativa) oil is notably high in linoleic acid, typically over half its fatty-acid content, alongside oleic and palmitic acids and the aromatic compound thymoquinone. Rosehip oil is similarly linoleic-rich, with additional linolenic acid.
Squalane is a different category again. It is not a fatty acid but a saturated hydrocarbon, closely related to squalene which the skin already produces. It is stable, non-comedogenic for most people, and mainly acts to soften and reduce water loss.
Reading a formula honestly
Ingredient lists are ordered by concentration down to one percent, after which order means little. A hero ingredient listed near the end of a long list is present in a token amount. Conversely, an oil blend with three or four recognisable lipids near the top is doing most of its work through those lipids, whatever the marketing emphasises.
Fragrance deserves a note. It is the most common cause of contact reactions in cosmetics, and in deeper skin those reactions frequently resolve into lasting dark marks. If your skin reacts easily, unfragranced formulas remove one large variable.
What oils cannot do
An emollient oil softens, smooths, and slows transepidermal water loss. It does not repair a genetic barrier defect, treat eczema or psoriasis, or lighten pigmentation. Products that claim otherwise are making a medical claim without the evidence that would justify it.
Used realistically — on damp skin, consistently, as part of a simple routine — a well-composed oil is one of the more reliable comfort tools available. That is a modest claim, and it happens to be true.