Paula's Choice has a review of the Obagi Clarifying Serum, which notes that while the vitamin C in the serum is an effective antioxidant, the product has other ingredients that might be irritating. The website gives the product a rating of "good."
According to Paula's Choice, the Obagi Clairifying Serum lists hydroquinone as its active ingredient. The serum also contains a form of Vitamin C known as L-ascorbic acid. This acid has antioxidant benefits, which can help get improve the appearance of skin discolorations. The serum has a pH of 2.5-3.0, which helps it to penetrate the dermal layers of the skin, but may be irritating.