OST C20 serum
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OST C20 serum

New name of this product is TIAM C source

OST C20 serum
OST C20 serum


30 ml
~14 USD


Ingredient analysis: Skincarisma

Water, Ascorbic Acid, Alcohol, Sodium Lactate, Butylene Glycol, Glucose, PEG-60 Hydrogenated Castor Oil, 1,2-Hexanediol, Phenoxyethanol, Sodium Hyaluronate, Bis-PEG-18 Methyl Ether Dimethyl Silane, Xanthan Gum, Diethoxyethyl Succinate, Ammonium Acryloyldimethyltaurate/VP Copolymer, Carthamus Tinctorius (Safflower) Flower Extract, Citrus Aurantium Dulcis (Orange) Peel Oil, Glycerin, Zinc PCA, Panthenol, Niacinamide, t-Butyl Alcohol, Camellia Sinensis Leaf Extract, Beta-Glucan, Tocopheryl Acetate, Lecithin, Caprylic/Capric Triglyceride, Ubiquinone, Diisopropyl Adipate

What is Vitamin C

Helps with collagen production
Acts as an anti-oxidant and restores and regenerates the skin

How to use it

Apply two or three drops to your entire face when you normally apply your serums, that is after toner and before moisturizer.

My thoughts on O.S.T Vitamin C serum (TIAM C source)

Before I bought this I heard so much about it. It really was a product everybody seemed to love, and really sold a lot. I decided to buy it to try and even my skin tone.

Used to fade acne scars and help with hyperpigmentation, even the skin tone and firming the skin

This product comes in a dark glass bottle with a dropper. The bottle is dark due to risk of oxidation. I feel that the best way to store it is in the box in the refrigerator and it will keep it from oxidizing too fast. The temperature and the lack of light will make it last a little bit longer.

Directly after applying it the skin will feel a little bit sticky, this goes away when you apply the rest of your products over it.

OST C20 serum
OST C20 serum

Is it okay to use my serum after it turned brown?

This is very much debated and opinions still differs on the matter. When Vitamin C oxidizes due to light or air it turns brown. What happens is that: the vitamin C molecule contains two electrons, when the first one is used (due to light or air exposure) the molecule becomes somewhat stable, when the second electron is lost, the product oxidizes and turns into dehydroascorbic acid. The browner your product get the more reaction it can cause your skin. So, I would definitely not recommend anyone using a brown serum, if its more on the orange side I would still use it.

When you buy this product fresh it is almost completely clear and the longer you have it, the product will become more orange and then finally brown. I would still use mine if it is slightly orange but not when it is dark in color. I guess it is for you to decide and some will say that you can’t use it as soon as it is a little orange and some will say it is fine. The decision is truly up to you.

I give this product 4/7


Where to buy it



Other brightening products

Skinfood Rice Brightening cleansing oil

TonyMoly Floria brightening peeling gel


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  • Emilia

    I’ve got C21.5 and I really like this serum, it helps me a lot of my pigmentation problems. I didn’t know that it’ll change colour to brown 🙂

    • Tess

      I have not tried the 21.5 version yet:-) I went for the cheaper one every time:-). If it hasn’t turned brown that is just perfect, then you got a good batch, and probably keeping it in a good place. If you have it in your moist bathroom it will probably turn brown. But hoping it will not:-)

  • Jade

    Hi! I have an awful acne breakout (large ones 🙁 ) on my left cheek and I don’t know how to make them go away. I read in many reviews that this product help. Is it okay to apply directly on acne spots? Or do I have to wait for the acne to heal before applying it? Thank you!

    • Tess

      Hi Jade, I am really not a dermatologist and can therefor only give you my personal recommendations of what I would guess is the best way.

      I would not use this product when you have a break out. This is to fade the scars that are there. This product will sting if you have open wounds.

      Instead I would really look into what products you are using today. Search in CosDNA on the products you are using, and try and learn what products giving you the outbreak to start with. CosDNA.com can give you great information on acne triggers. Are you using products for acne prone skin and do you use the right products for you skin type?

      Have you tried COSRX acne pimple master patch, they are supposed to be amazing on acne and they are not expensive. ( I haven’t tried them myself yet (I have super dry skin and seldom acne).

      This serum is great in fading the scars that are already on you face from earlier breakouts. Hope this information helps you:-)

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