We identify a segment of online reviewers (oracle reviewers), who systematically review successful products in early stages of a product’s life. Using two large datasets of online reviews for movies and books, we uncover oracle reviewers’ tendency to review successful products. We find that the larger the proportion of oracle reviewers in the crowd of product reviewers, the more successful the product is. We find that the mere appearance of oracle reviewers in the first week after product launch predicts product success, regardless of their opinion of the product. The proportion of oracle reviewers is predictive of product sales (or sales rank), overall number of product reviews and product lifetime. Finally, we identify characteristics of oracle reviewers such as their review frequency and writing style that distinguish them from other reviewers.