Olive leaf extract comes from the leaves of Olea europaea, not the fruit. That distinction matters, because the leaf and the oil have quite different compositions and quite different uses.
The principal constituent
Oleuropein is the compound of interest, a secoiridoid polyphenol that gives olive leaves and unripe olives their bitterness. It is present in the leaf at considerably higher concentration than in the fruit or the oil.
Hydroxytyrosol, a breakdown product of oleuropein, is also present and is itself a potent antioxidant.
How it works
Oleuropein is a strong free radical scavenger, with antioxidant capacity that compares favourably to vitamin E in laboratory measurement. It also chelates metal ions, reducing their capacity to catalyse oxidation reactions.
Research has additionally reported anti-inflammatory activity and effects on matrix metalloproteinase expression, the enzymes involved in collagen breakdown, which is why olive leaf appears in firming formulations rather than only as a general antioxidant.
There is also documented antimicrobial activity, part of the plant's own defence.
Leaf versus oil
Olive oil, INCI Olea europaea fruit oil, is an emollient rich in oleic acid. It works physically, softening the surface and reducing water loss.
Olive leaf extract is not an oil and has no emollient function. It is a water-soluble polyphenol extract used for antioxidant activity. The two share a plant and nothing else.
A useful practical note: oleic acid can be disruptive to the skin barrier at high concentrations, which is why olive oil is a debated choice for compromised skin. That concern does not apply to leaf extract at all.
Standardisation
As with all botanical extracts, oleuropein content varies substantially between suppliers, and the INCI name tells you nothing about it. Extracts standardised to a defined oleuropein percentage exist but are not identifiable from an ingredient list.
