Identity
Oxyresveratrol is a stilbene — a polyphenol closely related to resveratrol, with an extra hydroxyl group — found notably in mulberry (Morus alba). In cosmetics it is used under the INCI name Oxyresveratrol as a brightening antioxidant. The extra hydroxyl is not a trivia point: it is a large part of why oxyresveratrol is a markedly stronger tyrosinase inhibitor than resveratrol, which is its main cosmetic distinction.
Mechanism (as proposed)
Oxyresveratrol works on pigment primarily by inhibiting tyrosinase, the rate-limiting enzyme of melanin synthesis — and it does so potently, often cited as stronger than resveratrol and comparable to reference brighteners in assays. It is also a polyphenol antioxidant. The mechanism is genuine and, for a stilbene, unusually strong on the pigment axis. The honest limiter is physical: like other stilbenes it can be unstable and penetrate variably, so how much of that lab potency reaches living skin depends on the formulation.
Reading the evidence honestly
The gap is clinical: controlled human topical trials are limited, so oxyresveratrol's case rests more on potent in-vitro tyrosinase inhibition than on large skin studies — which is exactly why the grade is C rather than higher, despite a strong mechanism. As with every brightener in this register, the measured endpoint is the appearance of existing pigment, not a lightening of baseline skin colour, and persistent or hormonal pigmentation such as melasma is a dermatologist's domain.